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I would not recommend online services in case if you want to password-protect your sensitive or confidential PDF files from reading as it would not be a smart thing to upload your sensitive PDF files over these unknown third-party servers.
if you have Microsoft Word 2013, follow the procedure on
You can ZIP the PDF file and protect the ZIP file. You can also try programs like NitroPDF and PDF Encrypt ( http://www.mypdfsolutions.com/how-to-protect-lock-pdf-file.html ) to set the password protection to your PDF file without Acrobat.