it depends on what software you have, you will require either a full (licensed) version of adobes acrobat suite to be able to password protect a PDF (or a full licensed version of something like foxits PDF suite)
As far as I am aware there is nothing that will password protect a PDF for free (if anyone knows any different please say, as I have been looking for something to use for that at work!)
I don't know of anything that will password an existing pdf for free. But if you are creating it with (say) Open Office, you can password protect it at the same time.
If you want to password protect a PDF document, then you can use PDF security as this allows users to encrypt or decrypt PDF files which is protected by password.With the help of password remover function you can create a new PDF files from original file with no password which are restricted to change, edit, add annotation, print, copy, etc.
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