The file size should not exceed 10 megabytes. It should not be password-protected or protected with a permission password (also known as a master password). If a PDF file has embedded attachments, its signature may not be successful. There must be no special characters in the file name, and the name should not have more than 255 characters with its file extensions.
Some free programs and very old PDF conversion programs make an invalid, broken file format to be signed. In this case, you should copy the contents of a document to e.g. an office program and save it in PDF format.