Why will this PDF not open?

How it works

"This file cannot be opened" is the same message whatever the cause, which is why the advice you find is a list of things to try rather than an answer. The cause is usually visible in the first and last few hundred bytes of the file, and this tool reads them.

It takes any file, not just a PDF. That is deliberate. The most common reason a PDF will not open is that it is not a PDF: a download that hit a login page and saved the web page instead, or an image or a Word file renamed rather than exported. A tool that rejected those at the door would reject exactly the files worth explaining.

Four things get checked. Whether the file begins with a PDF header, and if not, what format it really is. Whether it ends with the end-of-file marker, because a PDF keeps its index of contents at the end, which is why a half-downloaded one fails completely instead of showing the pages that arrived. Whether the structure between the two holds together. And whether it is simply locked, which looks like damage and is not.

If the file turns out to be fine, that is an answer too, and the most common one. It means the problem is the app or the device you are opening it with, and the tool says so and tells you what to try.

Nothing is uploaded. Your file is read on your device, which matters here more than usual: a file you cannot open is a file you cannot check before sending it somewhere.