Print documents, if they are formatted correctly (e.g. double-spaced with good sized margins), they can be fairly easily annotated.  There are a variety of ways to annotate digital print documents.  Word processing tools like Microsoft Word and Google Docs have "comment" features that allow others to insert comments.

How you do it:

In both Microsoft Word and Google Docs you can chose Insert > Comments.  Make sure your cursor is near the word/sentence you want to comment on.

Tips and Suggestions:

• When using Microsoft Word, make sure you save your Word document in a format that is compatible with most versions of Word
• When using Google Docs, make sure to choose a color for your comments that is different then the comments of others.