The PDF standard provides for a means of preventing users from copying text from, or printing, a PDF, unless they have a password. The PDF content is encrypted as well, presumably to make it more difficult to get around. However, unless a user password is also specified, the decryption key is not kept secret from the viewer program, as otherwise not only wouldn't you be able to copy or print, but you also wouldn't be able to read it at all!
PDF is an open standard, so it especially shouldn't come as a surprise that there are tools to remove these restrictions without using a password. What does come as a surprise, however, is just how many PDF viewers actually enforce these restrictions, despite the fact that (in the case of commercial applications) it would give their program an advantage over most of the competition while actually requiring marginally less programming effort. Keep in mind most open-source PDF readers have it right, such as Evince, Okular, and the much more well-known PDF reader built into the Firefox browser.
Despite the presence of these alternative programs, however, there can still be plenty of value for wanting to remove the restrictions altogether. For instance, perhaps you want to send a PDF to somebody who might want to print it, but want them to be able to use whatever software they want for the task. In any case, this task can be performed by a program called QPDF, which is free and open-source. Keep in mind that if you need a password to actually read the PDF file, there is no easy way to remove the password. This is because in these cases, the encryption key actually is kept secret, being derived from the password given.
So here are the options you have for dealing with these restrictions, should you choose to disobey them:
Sample "protected" PDF's for testing and demonstration: