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My file ends in .pdf but the merge tool says it is not a valid PDF. Why?

Because the extension is only part of the file name, and the tool checks the content. A real PDF starts with a tell-tale marker in its first bytes, which [[merge-pdf]] looks for before trying to open the whole file structure. A renamed Word document, a photo saved with a .pdf name or a download that stopped half way all fail that check.

Published · 2 min read

Two different messages, two different causes

Read the wording of the message, because it tells you where the problem is.

  • 'Is not a supported file. Accepted types: PDF': the file does not start with the PDF marker at all. It is usually a document, an image or a saved web page whose extension someone changed.
  • 'Could not be opened as a valid PDF': the file looks like a PDF at the start, but its internal structure is damaged or incomplete, as happens with an unfinished download or a file copied from a failing memory card.

Cases that come up a lot

A shop owner receives invoice.pdf from a supplier, and it is really a Word file the supplier renamed to look like a PDF. Someone saves an ID photo from WhatsApp and names it id.pdf. Or a download link needs you to be signed in, so the browser saves the sign-in page itself under the file's name. In each case the name looks right and the content is something else.

The fix depends on what the file really is

  1. A Word or Excel document: open it in its own program, save or export as PDF, then add the new file.
  2. A photo with a .pdf name: rename it back to .jpg or .png and turn it into a real PDF with Images to PDF.
  3. A broken download or saved web page: download it again, make sure you are signed in, and check the size looks sensible rather than a few kilobytes.
  4. A file damaged at the source: ask the sender for a fresh copy, because the tool does not repair broken files.

Then add it to Merge PDFs alongside the files that were accepted. The good files stay in the list, so you do not need to add them again.

Common questions

The file opens in my reader, so why is it refused?

Some readers are forgiving and display what they can. If that is your case, save a fresh copy from your reader with print to PDF, then add that.

Why doesn't the tool accept Word files directly?

It merges PDF files only, and documents convert more accurately in the program that created them.

Does a refusal mean the file is dangerous?

No. It only means the content is not a sound PDF.