Jay, yes, nobody has updated any Keyczar code for almost year. Google was supposed to be working on Keyczar 2, but I don't know if that went anywhere.
Hancho, in terms of encrypting a PDF or PNG, what is your goal here? Are you writing something for fun or are you just trying to encrypt some files? If you just want to encrypt some files, Keyczar is probably not the right tool for the job.
If you're writing something for fun, the Python interface just handles strings. You would either have to buffer a whole file as a string or could use Jay's pull request. In terms of general Python crypto support, I haven't looked at it closely, by Cryptography.io is actively maintained:https://cryptography.io/en/latest/