Depends on what specifically you want your students to do in those PDFs and what do you mean by write .Annotations (comments). Most PDF reader apps will let them annotate the file (add comments), but while they all support the basic sticky notes, few go beyond. Modify PDF Online How To Edit Text In Adobe Modify PDF Online Pro Reader will give your students lots of drawing annotations, text editing markup, file attachments and more (some of it will require Reader-enabling in Modify PDF Online first), so it still sounds like the best free option. It will also let you add text with Fill & Sign tool and edit it, but neither it nor other free readers will let you edit and format the text and images already in the file. That's a feature of Modify PDF Online Pro and its other paid rivals.Forms. Those created with Modify PDF Online 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 Modify PDF Online 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 Modify PDF Online How To Edit Text In Adobe Modify PDF Online Pro Reader to type over it and place a drawing of a signature.Interactivity. This is tricky. Anything made with Flash or requiring it will only work in Modify PDF Online How To Edit Text In Adobe Modify PDF Online Pro Reader or Modify PDF Online Pro, only on desktop, only with Flash Player installed separately on the destination system. Not a reliable option anymore 26 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 immediately come to mind it's probably only video (.mp4, not .flv) and the showhide field functionality.