When you try to make files and/or folders available offline, you receive:
Offline files (\\server\share): Unable to make file Filename available offline on \\server\share. Files of this type cannot be made available offline.
The default settings exclude:
.db? .ldb .mdb .mde .mdw .pst .slmMicrosoft does NOT recommend altering the list, to prevent data loss.
NOTE: CSC (Client-Side Caching) can recognize when a synchronization issues exit, allowing you to select the server or client version, but it can NOT merge contents. .Pst is excluded to prevent conflicts with Outlook synchronization.
You can replace the list of excluded extension using the appropriate GPO (local, domain, OU, etc). To remove .pst from the list, you could navigate to:
Computer Configuration / Administrative Templates / Network / Offline Files / Files not cached
and enter *.slm;*.mdb;*.ldb;*.mdw;*.mde;*.db? in the Extensions box. Log off / Log on to replace the default list.
NOTE: To edit the local policy, run gpedit.msc. To edit the domain policy, use the Active Directory Users and Computers snap-in.