To protect PDF file you should have to use third party PDF Protection tool so that your important PDF file can be secure and protected. I can refer you a software link which can help you definitely to Protect your PDF file using password. http://www.sysinfotools.com/recovery/pdf-protected.html
My solution to protect PDF files is by adding DRM restrictions I tried this tool to disable copy paste in pdf files, it works very well in my opinion. http://www.pdf-no-copy.com
Thanks for sharing this wonderful tips, jpeg can also use by oCR software hence better work the way you told.