An apf file is a container for a digital ID. A digital signature is something you create using a digital ID just like a wet ink signature is something you create using a pen.
When you created your digital ID it must have been in Acrobat 5 or earlier because Acrobat 6 through 8 could only import the digital ID from an apf file, not create one. Beginning with Acrobat 9 we dropped support for apf files altogether (neither create nor import). In Acrobat 6 and later if you created a self-signed digital ID it was either stored in a pfx file or the Windows Certificate Store. So, any digital ID that was created in Acrobat 4 or 5 and saved in an apf file would have long since expired. It is possible that you need the digital ID to open an encrypted file, but as far as using it for creating digital signatures it would be useless. This begs the question, for what use are you looking to use the digital ID contained in the apf file?