Adobe ACROBAT READER 7.0 Manual page 154

Finding and adding existing digital IDs
If you created a digital ID file that does not appear in your list of digital IDs, you can
search for the missing digital ID file and add it to your list. One of the common encryption
methods that Acrobat uses, PKCS#12, has .pfx file extensions in Windows and .p12 in
Mac OS. Digital ID files from some earlier versions of Acrobat use an .apf extension. If
you select an .apf digital ID file, you may be prompted to convert the file to a supported
file type.
To find and add digital ID files:
1. Choose Document > Security Settings.
2. Select Digital IDs on the left, and then click Add ID.
3. Select Find An Existing Digital ID, and then click Next.
4. Click Browse, select a Digital ID, and click Open.
5. Type the ID password, and then click Next.
6. Click Finish.