If it can be printed, it can be doctored. I've done it as a joke, tabloids do it as a business method. Any half-decent retouching artist with a scanner, editing software and printer can do it. Unless you control the printing and use watermarking, holograms or some other anti-conterfeiting technique, this is not a technical question but a managment/business process one.

-- Or --

Hmm, you could always combine the key figures on the printout with some private key, make the MD5 hash and print that on the page. If the key doesn't match the result from the printed numbers, you'll know it's doctored.

Okay, so the first paragraph should read, "Just because it's in a bar code doesn't mean people can't forge it. If you can figure out how to print bar codes, so can they."