Sustainability of Digital Formats: Planning for Library of Congress Collections

America Online's (AOL) Personal Filing Cabinet or PFC is a proprietary database file that stores data from the AOL Desktop Software application to a local user-defined location for offline use. Stored data in PFC files can include stored email messages from AOL Mail (but not attachments to email), Internet bookmarks or Favorites, Away Messages, Newsgroup Postings and preferences such as user language and other configuration data. Because email attachments are not stored along with the message and instead are stored as 64-bit encoding, they can not be opened manually.

Each Username or AOL account has its own PFC file in the Organize folder. Because the file storage is local to the computer and to the Username, the stored files are not accessible if using a different computer, signing into a different account or signing in as a guest on the local computer. AOL Version 9 introduced the option to save email to AOL servers instead of "Save to My PC" which would provide access when signing in from another computer or as a guest from the local computer. Nonetheless, users often export their PFC data out of AOL to other platforms to improve accessibility and usability.

There is no published specification for PFC files; informal documentation comes from work done by Franz v. Gordon in 2002 through a reverse-engineering process and now available through the Internet Archive. Bruce Tomlin created partial documentation for the MacOS version of PFC which differs from the Windows version explored by Gordon. This document focuses on the Windows version. According to Gordon, PFC database files start with a header that includes the magic number file signifier, followed by an index table that contains pointers to all the data within the PFC. Each line in the index table starts with 52 53 00 00 (“RS”) and describes, among other values, the content type of the stored data object to which it’s pointing. The data could be a folder (including a system folder) or other coded values to identify the content type as a Favorite (bookmarked URL), a file that's been prepared for download or completely downloaded, email messages written or received, address book, email signature or availability status information ("Buddy away","Buddy preferences" and "Buddy icons"). Email messages are prefaced with 52 53 5c 30 5c 30 (“RS\0\0”) and then split into several subrecords starting with "AOLH" and ending with "AOLF". These subrecords include fields for the date/time, to, cc, bcc, subject, reply to, recipient, date/time as well as a flag to indicate if the message originated from inside or outside AOL. Other flag (at offset 0x16) indicates if the email message read (value 0), has an attachment (value 1), self written (value 2) and if the message has an embedded picture (value 3). Since they are not stored with the message, the name (which cannot include special characters) and length of attachments are recorded along with its stored location which varies for each browser. AOL 8.0 or 9.0 introduced a new option to send an email message as plain text so a value was added to the PFC to indicate text formatting preference as HTML or plain text. Content types other than email have different specific sets of relevant fields in the index tables.

Highly adopted thanks to the AOL Desktop market share. According to the Comscore Media Metrix Multi-Platform, AOL had 174,791 million monthly users in 2015. Because of the reliance on access to local data store, users often export PFC files to other formats including MBOX, EML and PST. Software tools such as Emailchemy can migrate PFC files to other email formats.

According to Gordon, email messages received with AOL 7.0 are stored in the PFC in encrypted form.