When you attempt to make files or folders available offline, the following error message may be displayed:
Offline files (\\server\share): Unable to make file Filename available offline on \\server\share. Files of this type cannot be made available offline.
By default, files with the following extensions are excluded:
.db? .ldb .mdb .mde .mdw .pst .slm
Microsoft does not recommend modifying the list of excluded files. See the "More Information" section for more details.
This behavior is by design.
Certain file types are excluded by default as a means to prevent file conflicts and data loss. The mechanism that the Offline Files feature uses is Client-Side Caching (CSC). CSC can recognize when synchronization issues exist (for example, if the copy of a file on the server has been modified since the last synchronization). In such a case, you can select which version to use, but you cannot merge the contents.
There may also be another synchronization mechanism that is typically used for the file type. One such file type is .pst files. This type is included in the list to prevent conflicts with the built-in offline and synchronization features in Microsoft Outlook. The Offline Files feature uses a replacement method of synchronization, so there is no way to merge the two copies.
You can modify the list of excluded extensions by editing the appropriate Group Policy object (local, domain, organizational unit, and so on). For example, to remove the .pst extension from the list of excluded files, enable the following sample policy in Policy Editor:
Computer Configuration\Administrative Templates\Network\Offline Files\Files not cached
Then, type the following string in the Extensions box:
Log off, and then log back on. This replaces the default list, preserving all but the *.pst extension. You can use the same method for any of the other extensions.
To edit the local policy, click Start, click Run, type gpedit.msc, and then click OK. To edit the domain policy, use Active Directory Users and Computers snap-in.