[cozzyd]

You can use the SHA-1 of the PDF's to avoid serving the same pdf twice.

[coulix]

SHA-256 ;)

[vgb2k11]

You've been away from HN a few days? The SHA-1 collision example uses PDFs in its demo. Hence other commenter saying SHA 256

[jhlgkhkhil]

The OP was almost certainly being sarcastic.

[logicallee]

For clarity can you edit your comment to add cozzyd (the OP you mention) - I am sometimes sarcastic but not in this case. I'll then delete this comment.