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16 min read Updated 9 Aug 2026Beginner

By , IT support technician

Understanding Document File Formats (.docx, .pdf, and More) ​

Applies to: Windows 11, Windows 10, Microsoft 365, Word, Excel, PowerPoint
Article Type: Informational
Last Updated: 2026-08-09

Summary ​

File extensions look like technical trivia and are nothing of the sort — they are the difference between a document somebody can work on and one they can only look at. This article explains what the letters after the dot actually mean, walks the formats you meet in a working week, shows you how to see them in the first place, and ends with the three-question rule for deciding what to send.

In this guide

Before You Start ​

  • None required. This is an explainer rather than a procedure — each practical fix links to the article that carries the steps.
  • The format examples are Microsoft 365 and Windows, because that is what most offices run. The ideas apply the same way on a Mac and in Google Workspace.

Instructions ​

1. Start With the One Idea Everything Else Follows From ​

A file name has two halves, and the short code after the last dot is the half that decides what happens when you double-click.

Q3 budget.xlsx and Q3 budget.pdf can hold the same numbers and are not the same file. The first is a spreadsheet — Excel opens it, recalculates it, and lets you change a figure in row 40. The second is a picture of the finished page. You can read every number in it and you cannot alter one.

That is the whole idea, and it splits every format you will meet into two families:

  • Working formats store the ingredients. Text is still text, numbers are still numbers, formulas are still formulas, and the app that owns the format can take the file apart and put it back together. .docx, .xlsx, .pptx, .csv and .txt are all in this family.
  • Delivery formats store the finished result. What is recorded is where every character sits on the page, which is why the file looks identical on every machine and why editing it is awkward. .pdf is the one you will meet most.

The extension is also how Windows decides which program to hand the file to. Which program that is, is a setting you control rather than something baked into the file — How to Change Default Apps in Windows 11 covers changing it and the Open with menu for one-off exceptions.

2. Turn On File Name Extensions Before Anything Else ​

Windows hides extensions by default, which means most people have never seen the thing this article is about.

With them hidden, Invoice in your Downloads folder could be a PDF, a Word document, a spreadsheet or a scan, and the only clue is a small icon. Turn them on and it says Invoice.pdf. The setting sits in a different place depending on which Windows you are running:

  • Windows 11: open File Explorer, click View on the toolbar, point at Show, and click File name extensions.
  • Windows 10: open File Explorer, click the View tab on the ribbon, and tick the File name extensions box in the Show/hide group.

Either way, every file name in every folder immediately gains its extension. This changes nothing about the files themselves — it is a display setting, and unticking the same box hides them again.

If the menu has moved in a Windows update, there is a route that does not depend on where the toggle lives: right-click any file, choose Properties, and read the Type of file line at the top of the General tab. It names the format whether or not extensions are on display.

One consequence worth knowing before you carry on: with extensions visible, renaming a file means keeping the part after the dot. Rename Invoice.pdf to Invoice January and you have taken its label off.

3. Meet the Editable Office Formats: .docx, .xlsx and .pptx ​

These three are where most work actually happens, and the letter on the end has a story behind it.

  • .docx — a Word document.
  • .xlsx — an Excel workbook, with its sheets, formulas and formatting intact.
  • .pptx — a PowerPoint presentation.

The x marks the formats Microsoft introduced with Office 2007. Underneath, each of these files is a compressed archive of XML text files describing the content — which is why they are smaller than the formats they replaced, and why a damaged one often behaves like a damaged zip file.

The originals are still out there. .doc, .xls and .ppt files open perfectly well in current Office, but they open in Compatibility Mode, which the title bar states beside the file name. In that mode a set of newer features is switched off so the file stays readable in the old apps. To bring one up to date, click File > Info > Convert and save. Keep a copy of the original if anyone in the chain is still running Office 2003, and understand what the conversion does: it rewrites the file in the modern format, and the old one is replaced rather than kept alongside.

You will also meet .docm, .xlsm and .pptm occasionally. The m means the file carries macros — small automated routines — which is why many email systems block them outright and why Office opens them with the macros disabled until you choose otherwise.

4. Understand Why a PDF Behaves Like Paper ​

A PDF is a description of a finished page, and everything people find strange about it follows from that.

The Portable Document Format — created by Adobe and now an open standard — stores the position of every character, the fonts themselves, and the images, all locked to the page. Open it on any computer, in any PDF reader, with any printer attached, and it looks the same, because there is nothing left for the receiving machine to decide.

That is exactly why it resists editing. In a Word document there are paragraphs that reflow when you add a sentence; in a PDF there are marks placed at coordinates. Adding a word means shifting everything after it by hand.

So a PDF is the right choice when the file has to be read exactly as designed — contracts, invoices, proposals, anything going to a client, anything being printed at the other end, anything where the page numbers matter. It is the wrong choice for anything a colleague has to work on with you.

Two things about PDFs surprise people, and both are worth knowing:

  • Word can open a PDF and rebuild an editable document from it. It works well on plain, text-heavy pages and less well on multi-column layouts and forms. How to Convert a Word Document to PDF (and Back) has both directions.
  • A scanned PDF is a photograph of a page. The words in it are not text yet — they are pixels arranged in the shape of letters — so searching and copying find nothing until the file has been through text recognition.

5. Know What .csv Throws Away ​

A CSV file is a table stripped back to the values, and it loses three things that people expect to survive.

.csv stands for comma-separated values, and the file is exactly what it sounds like: plain text, one row per line, commas between the columns. Open one in Notepad and you can read the whole thing. That plainness is its purpose — a CSV is the format that banks, payroll systems, online shops and CRMs all agree on, because there is no version, no licence and nothing proprietary to disagree about.

The price is that a CSV holds values and nothing else. It cannot carry:

  • Formatting — no bold, no colours, no column widths, no cell borders.
  • Formulas — a cell containing =SUM(B2:B40) is saved as the number it currently shows. The formula behind it is gone.
  • Multiple sheets — a CSV is one table. Save a twelve-tab workbook as CSV and you keep the tab you were looking at.

Excel warns you once when you save this way, and the warning is real. Use Save a Copy or Save As with a new name rather than converting your only file: once you close the workbook, the formulas, formatting and other tabs are not recoverable from the CSV, and you would be rebuilding them from memory.

Opening one has its own trap in the other direction. Excel inspects each column as it imports and converts what it thinks it recognises, which is how account numbers lose their leading zeros and product codes turn into dates. How to Open a CSV File in Excel Without Breaking It has the import route that stops it happening.

6. Recognise the Text Formats and the Ones From Outside Microsoft ​

Four more formats turn up regularly, and none of them needs more than a sentence of explanation.

  • .txt — text and nothing else. No fonts, no bold, no images, no layout. Notepad opens it, so does everything else ever written. Useful precisely because nothing can reformat it: notes, logs, a list of values, anything you want to survive untouched.
  • .rtf — Rich Text Format, the middle ground. It carries bold, italics, basic fonts and simple tables, and opens in almost any word processor on any operating system. Largely superseded by .docx, but it still appears where a file has to open somewhere unpredictable.
  • .odt, .ods, .odp — OpenDocument formats, native to LibreOffice, OpenOffice and several government systems. Word, Excel and PowerPoint open and save all three. Plain documents cross cleanly; complex layouts drift, in the same way they do between Office versions.
  • Google Docs, Sheets and Slides — the odd ones out, because there is no file on anyone's computer. A Google document lives in Google Drive, and what gets shared is a link to it rather than a copy of it. To turn one into a file you can keep, open it and use File > Download, choosing .docx, .xlsx or .pdf. If someone sends you a Drive link and you need an attachment, that download is what to ask them for.

7. Tell the Image Formats Apart: .jpg, .png, .heic and .svg ​

The differences between image formats come down to what each one is willing to throw away.

  • .jpg (also written .jpeg) is lossy. It shrinks a photograph by discarding detail the eye is poor at noticing, which makes it small and makes every re-save a little worse than the last. Right for photographs, wrong as a working format for anything you edit repeatedly.
  • .png is lossless. Nothing is discarded, files are larger, and it supports transparent backgrounds. Right for screenshots, logos, diagrams, and anything containing text or hard edges — a screenshot saved as JPEG comes out visibly smeared around the lettering.
  • .heic is Apple's High Efficiency format, and it is what an iPhone saves photos as unless it is told otherwise. It produces smaller files than JPEG at the same quality, and Windows needs an extra component before it will show them. Apple's own answer is a setting on the phone's camera rather than anything on the PC. How to Open HEIC Photos on Windows covers both that switch and the Windows component that displays the files you already have.
  • .svg is vector. Rather than a grid of pixels, it stores the instructions for drawing the shapes, so it scales from a favicon to a billboard without ever going blurry. Logos and icons arrive in this format; any web browser opens one.

The short version: photographs are .jpg, screenshots and logos are .png, and anything that has to be resized is .svg.

8. Treat .zip as a Box Rather Than a Document ​

A zip file is not a document at all — it is a container holding other files, compressed to travel as one attachment.

Double-click a .zip in File Explorer and Windows shows you what is inside, laid out like a folder. That display is convincing and slightly dishonest: you are looking into the archive, not at real files on your disk. Open a document from in there, edit it, and the changes have nowhere to be saved back to.

So extract first. Right-click the .zip, choose Extract All, and confirm the destination. Windows writes a normal folder of normal files beside the archive, and the .zip itself stays exactly as it was — extracting copies rather than moves, so there is nothing to undo if you picked the wrong destination.

.rar and .7z are the same idea from different tools, and Windows cannot open either without extra software. If someone sends you one, asking for a .zip instead is a reasonable request.

9. Understand Why Renaming a File's Extension Does Not Convert It ​

Changing the label on a box does not repack the contents, and this is the single most common misunderstanding about file formats.

Rename report.docx to report.pdf and you have a Word document wearing a PDF label. Nothing inside it changed. Windows asks first — "If you change a file name extension, the file might become unusable" — and that warning is describing exactly what will happen next: your PDF reader opens the file, finds Word's internal structure where a PDF's should be, and reports that the file is damaged or cannot be opened.

Nothing is lost, though, and this is worth knowing before you panic. Rename it back to .docx and it opens perfectly, because the contents were never touched. The same applies to a file you renamed weeks ago and forgot about.

There is one legitimate use for renaming an extension: a file that arrived with the wrong one or none at all, where you know what it really is. A .txt that is plainly a spreadsheet's worth of comma-separated data becomes usable as .csv. That is correcting a mislabelled box, not converting it.

To convert for real, use the app that owns the format: Save As or Export in Word, Excel or PowerPoint to write a new file in the target format, or File > Open in the app you want to end up in. The step-by-step for the commonest pair is in How to Convert a Word Document to PDF (and Back).

10. Choose a Format to Send Using Three Questions ​

Every version of "what should I attach?" resolves once you ask what the person at the other end has to do with it.

  • Do they need to edit it with you? Send the editable format — .docx, .xlsx, .pptx — and send it as a link from OneDrive or SharePoint rather than as an attachment. One file, one place, no versions crossing in the post.
  • Do they only need to read it, exactly as you designed it? Send a PDF. Their fonts, screen, printer and Office version stop mattering the moment you do.
  • Is it going into another system rather than to a person? Send .csv, or whatever the receiving system's own instructions ask for. Import screens are specific, and their documentation beats guesswork.

Two extras that settle most of the remaining cases. If the recipient named a format, that is the answer — an accountant asking for CSV is telling you what their software imports. And if a file has to be both read exactly and edited later, send both: the PDF for signing off, the editable file for the next round.

Troubleshooting ​

TIP

When you cannot tell what a file is, right-click it and choose Properties. The Type of file line on the General tab names the format in words, and it works whether or not extensions are on display and wherever Microsoft moves the menus next.

Symptom / ErrorPotential CauseSolution
You cannot tell what kind of file something isWindows hides file name extensions by defaultTurn them on (Step 2), or right-click the file, choose Properties, and read Type of file on the General tab.
"If you change a file name extension, the file might become unusable"You edited the part after the dot while renamingClick No and leave the extension alone. If you already changed it, rename it back — the contents are untouched. Convert with Save As instead (Step 9).
A .csv opened in Excel has lost leading zeros or turned codes into datesExcel converts each column as it importsUse the import route in How to Open a CSV File in Excel Without Breaking It, which lets you mark those columns as text.
Photos from an iPhone will not open on WindowsThey are .heic files, which Windows does not display out of the boxInstall the Windows component or switch the iPhone camera to Most Compatible — How to Open HEIC Photos on Windows has both.
The title bar says Compatibility ModeThe file is still in the pre-2007 .doc, .xls or .ppt formatClick File > Info > Convert to rewrite it in the modern format. Keep a copy first if anyone in the chain runs Office 2003.
A file opens in the wrong program when you double-click itThe default app for that extensionThe file is fine — the association is a Windows setting. See How to Change Default Apps in Windows 11.
Edits to a document opened from inside a .zip disappearThe file was opened from within the archive rather than extractedRight-click the .zip, choose Extract All, and work on the extracted copy. The archive is left intact.
A colleague cannot open the file you sentTheir software is older, or does not read that formatAsk which formats they can open, then send .pdf for reading or .rtf and .csv as the near-universal editable fallbacks.