Under "common problems" would have to be "trouble viewing PDFs inside a browser." This has created endless threads in numerous forums, for all pdf makers--not just PDFXChange, so expect your share here. But a short list of steps like I was given in this thread would be great: viewtopic.php?f=35&t=7860
As someone else has suggested, look at Bikkhu's page. This would be nice, as I can guarantee you that most folks will not realize that a PDF viewer can have customizable toolbars, like he demonstrated you can do.
Also, I recieved help from him in changing the default text/formatting for my markups on pre-existing pdfs. That is also a major plus of PDFXChange and should be emphasized in a KB article.
Also, pinning recent files should be in there (a short one, but just to let them know you can do this).