Why your PDF will not open

A file that will not open produces the same unhelpful error whatever the cause. Working through the possibilities in order of likelihood usually finds it in a couple of minutes.

It did not finish downloading

By far the most common cause. An interrupted transfer leaves a file that has a name, a size and a sensible-looking icon, and is missing its end.

PDFs are particularly prone to this because the index that tells a reader where everything is sits at the *end* of the file. Lose the last few kilobytes and nothing can be located, even though most of the content is present.

Check the file size against the source if you can. Otherwise, download it again, ideally over a different connection.

It is not actually a PDF

Files get renamed. Someone changes a .docx to .pdf believing that converts it, or a download saves an error page as a .pdf without anyone noticing.

Open it in a plain text editor and look at the first line. A real PDF starts with the characters %PDF- followed by a version number. If you see HTML, or a wall of unrelated binary, the extension is lying.

It is encrypted

Some viewers report a password-protected file as damaged rather than prompting for the password, which is unhelpful but common. If the document came from an organisation that protects its documents, try opening it in a different reader to see whether a prompt appears.

It is genuinely damaged

Cross-reference tables get corrupted. Storage fails. Files get truncated by a full disk or a crashed application partway through a save.

This is where repair helps. A tolerant reader can often parse the objects in the file while ignoring the broken index, then write out a fresh file with a correct one. It works when the content is present but the map to it is wrong.

What repair cannot do is invent content that is not there. If the file was truncated, the missing part is gone, and a repaired version will simply be shorter. Always check the page count of the result against what you expected.

It is too large for the viewer

Less common, but real on mobile. A very large document can exhaust the memory available to a viewer, which then reports a failure that looks like corruption. Trying it on a desktop machine distinguishes the two.

Before you conclude it is broken

Try a second viewer. Browsers, operating system previewers and dedicated readers all have different tolerances for malformed files, and one will frequently open what another refuses. If a file opens anywhere, it is recoverable, resave it from the viewer that managed it.

Working through it in order

The checks are quick, and doing them in order of likelihood saves time.

Try a second viewer first. Browsers, operating system previewers and dedicated readers all tolerate different kinds of malformation. If any of them opens the file, it is recoverable: resave it from whichever one managed.

Check the size. A file noticeably smaller than expected was truncated. Compare against the source if you can.

Check the first bytes. A real PDF begins with the characters %PDF- and a version number. Open it in a text editor and look. HTML at the top means you saved an error page.

Check for a password prompt in a different reader, since some report encryption as damage.

What repair does and does not do

A repair tool parses whatever it can while ignoring the file's index, then writes a fresh file with a correct one. That fixes damaged cross-reference tables, stale offsets, and files that were appended to badly.

It cannot recover content that is not present. A truncated download is missing bytes, and no tool invents them. The repaired file will simply be shorter, which is why checking the page count of the result against what you expected is the step people skip and should not.

Preventing the common causes

Download over a stable connection and check the size afterwards. Most truncation happens on flaky mobile connections and gets noticed weeks later.

Do not rename files to change their type. Renaming a .docx to .pdf changes nothing about its contents.

Keep an eye on disk space. A save interrupted by a full disk produces exactly this failure, and the file looks normal until someone opens it.

When the file matters

If a damaged document is important and repair does not recover it, stop trying tools and go back to the source. The person or system that produced it almost certainly still has it, and ten minutes of asking beats hours of recovery attempts on a file that is missing its second half.

Files that open but are clearly wrong

A separate category worth naming: the document opens, but the text is garbled, the fonts are substituted, or the layout has collapsed.

That is usually missing embedded fonts rather than damage. The file is intact; the machine reading it lacks something the file assumed would be present. Opening it on the machine that created it, or asking for a version with fonts embedded, resolves it.