Hi Elpie, what kind of files were they originally?

Did Firefox change the file extension to .pdf, but it's still really a .doc or .jpg, etc., or does the file now open normally in a PDF reader?

All files like excel, word, etc...they are converted to pdf not just changing the extension. the file does not open normally in a pdf reader...

I guess I asked that in a confusing way. If they are converted to PDF, will Acrobat (or whatever you use for reading PDFs) open them?

If not, did you try changing the file extension back to the correct one? You may need to change the Windows setting that hides common file extensions.

https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/