How do you shrink a PDF file?

PDF (file format): How do adobe reader get the content without knowing the password?

• Recently, I'v got a PDF file from my friend, I opened it up with my Adobe Reader X, and I see a SECURED note on the title bar. I find the properties of this file, it says that the security method of this file is "Password Security" and the PDF version is 1.7 level 8. I can't edit it without knowing the password to this file. Since the reader X opened the file without asking me a password, the content should not be encrypted, the reader X is just following a protocol which tell it not allow people to edit the file. So if someone write a program that did not follow the protocol, the password will be useless. Then I was confused about this problem, is it possible to write that program? If not, I must be wrong and the content of the pdf must be encrypted. But another problem is: How do Adobe Reader get the content without knowing the password? Thanks a lot. PS: I found some PDF password remove program, and try them on this PDF file, none of them works, and that is why I put the version of the file format in the question. Plus, I can open this file with only Adobe Reader X and later. So I think Adobe has change the encrypt algorithm. I've asked same question on stackoverflow http://bit.ly/JdJI2e http://bit.ly/JdJI2e

• Answer:

  PDF files that enforce do not edit and do not print are just flags. A reader can be chosen to ignore then. I actually recompiled evince to ignore this flag once.