If the documents aren't in constant use, the most secure way to encrypt them is with a userland program like PGP. Userland crypto knows where files begin and end, and can store metadata to improve the encryption. They can provide cryptographic integrity --- far more powerful than the incidental integrity check Bitlocker tried to provide, or the virtually zero integrity that XTS provides. They're randomized, so the ciphertext can have semantic security; it reveals nothing at all about the plaintext, even as the files are edited in place under the same key.
Sector crypto can't do anything even approximating this without contortions like geli.
If I was trying to protect files from nation state adversaries, I would not consider Truecrypt.
That doesn't mean I think you shouldn't run something like Truecrypt. I think you're better off with whatever your OS provides, but some kind of sector-level crypto, be it Bitlocker, Truecrypt, or Filevault, is still useful.
But if you're serious about protecting a specific set of files, encrypt them manually, no matter what else you do.
