Unlock PDF is a free online tool that helps you get rid of annoying passwords in few seconds. It can unlock most of the password protected PDF files (where printing, viewing etc is restricted/protected by password). In some cases, especially, if the pdf is strongly encrypted, you would need to tell us the password as well, to finally produce the unlocked version of the pdf.
Using this tool doesn't require any registration or signup. It's completely free and it does not add any watermarks/ads to the generated pdf. However, the file size for input files are limited to 32 MB. You're also recommended to use a modern web browser (Firefox or Google Chrome).