RSA doesn't have collision, but usually what's encrypted isn't the actual document, it's the hash of the document. Since asymetric crypto is too slow usually they just take a hash of the doc and then encrypt the hash with RSA with private key. Then the end user uses public key to decrypt hash, takes hash themselves of the document, and compares hashes. If they match this means document hasn't been changed. If MD5 (the most common hash) has collision (still very rare I think) then that is why you may see it because possibly they are RSA the hash only.

-Lunar