If you have a downloaded "donor" save, you must copy bytes 64 to 564 from that file and overwrite the exact same range in your own User.profile to maintain your account's "hash" while gaining the donor's progress. Full Story Summary
F.E.A.R. 3 is notorious for its lack of reliable Steam Cloud support. Many players lose progress after reinstalling because the game does not automatically restore files from the cloud. To prevent losing your 100% progress again, it is highly recommended to manually backup the 21100 folder to an external drive. Steam Community hex editor software recommendations or a more detailed breakdown of the Rank 21 rewards fear 3 save game 100 work