Depends on what specifically you want your students to do in those PDFs and what do you mean by write. Annotations ments) . Most PDF reader apps will let them annotate the file (addments) but while they all support the basic sticky notes few go beyond. Adobe Reader will give your students lots of drawing annotations editing markup file attachments and more (some of it will require Reader-enabling in Acrobat first) so it still sounds like the best free option. It will also let you add with Fill & Sign tool and edit it but neither it nor other free readers will let you edit and format the and images already in the file. That's a feature of Acrobat Pro and its other paid rivals. Forms . Those created with Acrobat Pro (acroforms) can be filled in most readers including on mobile. Depending on how the form was created you may need to Reader-enable it in Acrobat so it can be saved or digitally signed. If your PDF looks like a form but is in fact flat without any fields e.g. just for printing you can still use the Fill & Sign tools in Adobe Reader to over it and place a drawing of a signature. Interactivity . This is tricky. Anything made with Flash or requiring it will only work in Adobe Reader or Acrobat Pro only on desktop only with Flash Player installed separately on the destination system. Not a reliable option anymore Almost anything that requires JavaScript won't work on mobile. There are very few PDF interactivity features that work more or less universally. Of those that immediatelye to mind it's probably only video (.mp4 not .flv) and the show field functionality.