PDF is a great way to secure your documents, but -- if you have Adobe Acrobat -- there are also a number of ways you can make your materials more interactive and adaptable in PDF format. In turn, this makes PDF a far more versatile format for learning resources to be uploaded into LMS set ups like Moodle - where they can become stand-alone assignments to be downloaded and completed without needing access to the web.
The tutorial below demonstrates how to embed text form fields into your PDF document so that learners can submit answers to questions:
You might also like to look at embedding audio help files into your PDF resource so that learners cna get additional instructions, tips or advice on how to do something:
I've also demonstrated in the past how to do this audio file embedding with materials for younger learners as well (and as part of generating a resource that might be presented for the class on a beam projector, for example):
Hope these give you some ideas on how to go those extra yards with your PDF documents to help make them more multi-modal resources with more chances of teaching/learning interaction.