Anyone know of a good Windows file compare program?

Would like to compare two files and see if they are identical.  We are transmitting pdf's across a vpn and suddenly when they get to the destination Adobe Reader will not open them.  Source files open readily.  Would like to be able to prove yes or no that the files are being corrupted in transit or if it is something else going on.

Preference would be to be able to see the differences in hex, but a listing of the differences would do.

Obviously, would prefer free, but would consider a solution which is not.

This is for a Windows environment, I see that there are a few things out there for unix.

Also if anyone has any ideas as to what might be going on, would love to hear them.  This is quite strange...