Before trying anything else, do this: drag the file into a browser tab. Chrome, Edge, Firefox, and Safari all have capable built-in PDF viewers, and they are tolerant of malformed files that stricter readers reject outright.
If it opens in the browser, the file is fine and your reader is the problem. If it does not, the file is genuinely damaged or is not a PDF at all. That one test splits the possibilities in half and takes five seconds.
Read the error message — it tells you which problem you have
"Damaged and could not be repaired" — the file is truncated or corrupt.
"Not a supported file type" / "Failed to load document" — this is often not a PDF at all, despite the extension.
A password prompt — the file is encrypted. Nothing is broken.
Opens, but pages are blank — the file is fine; this is a rendering problem.
Opens with boxes or garbled characters — fonts, not file integrity. See troubleshooting PDF fonts not displaying.
The file is truncated or corrupt
Most often caused by an interrupted download, an email attachment that did not transfer completely, or a file copied from failing storage.
Check the size first. A PDF of 0 bytes, or a few hundred bytes, never downloaded properly. This is the single most common cause and takes one glance to confirm.
Download it again, ideally from a different network or browser. Interrupted transfers are more common than people assume, particularly on flaky mobile connections.
Look inside the file. Open it in a plain text editor. A valid PDF starts with %PDF- followed by a version number in the first line. If the top of the file is HTML, you downloaded an error page or a login redirect rather than the document — very common with links behind an expired session. If it starts with PK, it is a ZIP-based file such as a .docx that has been misnamed.
Try a repair. qpdf can often reconstruct a damaged cross-reference table, which is the most frequent form of corruption:
qpdf --replace-input damaged.pdf
Or write to a new file with qpdf damaged.pdf fixed.pdf. Free online repair services exist too, though they upload your document. Recovery is genuinely partial — a truncated file is missing data that no tool can invent. See how to recover a corrupted PDF.
Ask for it again. Faster than any of the above, and usually the right first move.
It is not really a PDF
Files get renamed, and mail systems occasionally mangle attachments. If the header check above showed something other than %PDF-, ask the sender what they actually exported. A .docx renamed to .pdf will never open in a PDF reader.
Note that Windows hides file extensions by default, so report.docx.pdf may display simply as report.docx. Turn extensions on if you deal with this often.
It is password-protected
A password prompt means the file is intact and encrypted. You need the password — there is no legitimate way around a user password, and modern PDF encryption has no back door.
Institutional senders often use a formula stated in the covering email: date of birth as DDMMYYYY, the last four digits of an account number, a national ID. Check the email you skimmed. See how to remove a password from a PDF.
Note that some editors refuse encrypted files even when you know the password, because they cannot prompt for it. Ours included: open it in a reader with the password, print it to a new PDF, and edit the unprotected copy.
It opens but the pages are blank
The file is not broken and this is worth distinguishing, because people delete files that were fine.
Very large or complex pages may take a while to render, particularly high-resolution scans on an older machine. Wait, then scroll.
Older Adobe Reader with hardware acceleration occasionally renders blank pages. In Reader, turn off "Use 2D graphics acceleration" under Preferences → Page Display.
XFA forms. Some government and banking forms use Adobe's XFA technology, which browsers and most readers cannot render. You will typically see a message asking you to open the file in Adobe Acrobat Reader — that message is accurate, and Reader is free. See XFA forms explained.
Content on hidden layers, occasionally used in print production.
It is the reader or the browser, not the file
If the browser test worked but your usual reader fails:
- Update the reader. PDF is an evolving standard and old software genuinely cannot parse newer files.
- Try an incognito window, which disables extensions. PDF-related and security extensions are a frequent culprit.
- Clear the browser cache, which sometimes holds a partial copy of the file.
- Check disk space. Readers need somewhere to write temporary files, and a full disk produces confusing errors.
- Restart. Genuinely fixes a proportion of these.
A note on unexpected PDFs
If a PDF arrived unexpectedly and will not open — particularly one urging you to enable something, click a link, or enter credentials — treat that as a signal rather than a malfunction. PDFs can carry embedded JavaScript and links, and malformed documents that crash readers have historically been used to exploit them. Do not troubleshoot a document you were not expecting from a sender you cannot verify. Delete it and confirm with the supposed sender through a channel you already trust.
The order to try things
- Open it in a browser tab.
- Check the file size, and check the first bytes are
%PDF-. - Download it again.
- Ask the sender to resend or re-export.
- Try
qpdf --replace-input. - Update or switch readers.
Steps 1 to 4 resolve the overwhelming majority of cases, and step 4 is often faster than steps 2, 3, and 5 combined.
