My first question is:
Are you SURE that the document hasn’t been security protected?
When creating PDF files (at least in Adobe Acrobate) we have the ability to restrict the use of the pdf…we can disable printing, editing, copying…etc.
If you’ve gotten the pdf from the internet or from an unknown source, they may have purposely restricted your ability to print or otherwise alter the document. In this case, without a password, you’re not going to get anywhere.
This is a very common practice for authors who want to give their work some level of permanence and security and protect their copyright ownership.