I wonder if I can open a DRM protected PDF file with an open-source utility. Is it enough to type the password when requested (command line parameter)?
PDFs can have two passwords: one for viewing the PDF, other to remove protections like copy-protection, etc.
From the tags of your question I assume you want to remove the protections like copy-protection, etc. And I assume that you know that you shouldn't circunvent copyright, etc etc.
If your open-source utility is able to accept both passwords, so yes, you could remove the protection to be able to copy something from the pdf. In general those programs just accept one password, the viewing one.