Why your PDF will not open — common causes and how to fix them

A document you need will not open. The reader reports it as damaged, or shows a blank window, or simply does nothing. Before assuming the file is lost, it is worth working out which problem you actually have — because the fixes are completely different and most of these situations are recoverable.

First: check the file size

This takes five seconds and immediately rules out the worst case. Look at the file size on disk. If a document that should be a twenty-page report is only a few kilobytes, the download was truncated and most of the content never arrived.

No repair tool can recover data that does not exist. If this is your situation, stop and get a fresh copy — re-download it, or ask the sender to send it again. This is by far the most common cause of an unopenable PDF and by far the easiest to fix.

Always try downloading the file again before anything else. A large proportion of broken PDFs are simply interrupted transfers.

Cause 1: the transfer was interrupted

A download that stopped at 90%, a USB drive pulled out mid-copy, a sync client that uploaded a half-written file, or an email gateway that mangled the attachment. The file on your disk is incomplete.

The fix is a fresh copy. If the source is gone and you only have the partial file, a repair tool may recover the pages that did arrive, which is often better than nothing.

Cause 2: the index is damaged

This is the interesting one, and the reason repair works at all.

A PDF is not one continuous stream. It is a collection of objects — pages, fonts, images — plus an index at the end recording where each object lives. Damage that index and the whole document appears unreadable, even though virtually all the content is still present and intact.

That is why so many corrupt PDFs are recoverable: usually the pages survived and only the map to them is broken. A repair tool scans the file for surviving objects and builds a fresh index around whatever it finds.

Cause 3: it is encrypted, not broken

Some readers give unhelpful errors for password-protected files, particularly when the document is opened by another program rather than directly. If the file came from a bank, a payroll system or a government service, encryption is very likely.

Check the original email — it usually explains the password scheme. Common ones are a date of birth as DDMMYYYY, a postcode, or the last four digits of an account number. Once you can open it, removing the password makes the file workable with other tools.

Cause 4: your reader is stricter than the one that made it

PDF readers vary considerably in how strictly they parse the format. Chrome's built-in viewer is forgiving; Acrobat is strict. A slightly malformed file — perhaps produced by an unusual export tool — can work perfectly in one and fail in the other.

This explains the maddening situation where a colleague insists the file is fine and it will not open for you. Try a different reader first: open it in a browser by dragging the file into a tab. If it displays there, the file is largely intact and running it through a repair tool produces a clean version that satisfies stricter readers.

Cause 5: it is not actually a PDF

Occasionally a file has a .pdf extension but is something else entirely — an HTML error page saved during a failed download, or a file renamed by someone who assumed changing the extension would convert it.

Opening it in a plain text editor is a quick diagnostic. A genuine PDF starts with the characters %PDF followed by a version number. If you see HTML tags or something unrecognisable instead, the file was never a PDF and no repair tool will help.

A working order to try

1. Check the file size for obvious truncation.
2. Download or request a fresh copy.
3. Try opening it in a different reader, such as a browser tab.
4. Check whether it is password-protected, and find the password.
5. Run it through a repair tool.
6. Verify every page of the repaired file, not just the first.

That final step matters. Repair recovers what it can find, which is occasionally most of a document rather than all of it. Scroll the whole file before you delete the broken original or send the recovered version to anyone.