The file extension APF is used by older versions of the Adobe Acrobat document file software. These files are used to add a digital signature to a PDF document. In this way, it can be determined later, for example, who has made which change to the document.
These profiles or certificates can be loaded under the security setting under 'Edit -> Protection' and are usually password protected. Also with the Acrobat Reader such files can be attached, for example for the writing of comments in pdf documents.
Newer versions of Acrobat use a different format for these profile files with the extension pfx or P12 (MacOS).