Here's some small things that I have noted when comparing Foxit Reader and PDF-XChange:

You are able to add annotations with both, but the free version of Foxit Reader leaves an evaluation mark on the annotated page when you save the document whereas the free version of PDF-XChange does not.

Foxit Reader supports a type of text-hilighting that PDF-XChange does not: Squiggly underline. Nonetheless, PDF-XChange does display the squiggly underline both within the PDF as well as in its "Comments List"; however, PDF-XChange is unable to display text that you have used for describing the squiggly itself (and you are not able to add any text nor are you able to delete this type of annotation from the document). This may or may not cause problems (I have been using squiggly hilighting quite a lot in Foxit Reader, which quickly becomes an issue if one wishes to migrate to PDF-XChange viewer). The "Caret" markup tool is also missing in PDF-XChange. The caret is displayed in PDF-XChange but behaves in the same way as the squiggly underlines. Not having these markup utilities may also cause problem if you are asked to review PDF:s that have these types of comments/annotations.

PDF-XChange has a so-called Comments List whereas Foxit does not. The Comments list enables you to see where a PDF-document is annotated and allows you to quickly navigate to an annotation. But, it is still a cheap knockoff of Adobe Reader's comments list, which not only allows you to see how annotations are distributed within a document but also allows you to see the texts stored in each annotation.

Foxit Reader (as well as Adobe Reader) allows one to view a text of a pop-up note by simply hovering over the annotation whereas PDF-XChange does not. I think that such a feature is nice when you are reading PDFs. But, Foxit has a problem in that it will truncate longer texts before displaying it in the pop-up, which makes it much less useful than Adobe Reader in this department.

The typewriter tool exists in both tools. I use this tool quite extensively, but it is difficult to manage in PDF-XChange when typing longer texts. If you have a long text on one line you may decrease the width of its container to make the text in it split on multiple lines. But, if you try to edit the text in that container again, the container will resize itself automatically so that the text is once again shown on a single, very long line. Also, the typewriter tool in PDF-XChange viewer seems unable to remember what font I used with it, which is very annoying (I don't like courier new, it is ugly!!!!).

Why would you want to name your company Tracker Software? :huh: Makes me think about spyware and those sort of things. Does anyone know if PDF-XCHANGE VIEWER is free from that kind of stuff?

I bought Foxit Reader 2.1 as I thought it seemed like a good deal. But, alas, I find that I cannot trust the software entirely. I have experienced (once now, but only annotated a few PDFs) that the annotations made in Foxit does not show in Adobe Reader 8.0.0 and 8.0.1. Not only that, but Foxit left the PDF with annotations in it that were visible (but empty) in Adobe Reader but invisible in Foxit. Adobe Reader also gave me a error message: "Wrong number of arguments for a setcolor operator". Has anyone else here had this experience? I don't know if I can trust the software enough to continue using it. Worst case scenario is that Foxit Reader would make it impossible for me to move to my annotated PDFs to another annotating system if I wanted to as it seems to have issues with portability.

I wrote to Foxit's support about this problem a while ago in the hopes that they could (at least) give me instructions on how to avoid this while waiting for a new version of Foxit. This is what I wrote:

Hi,

I made comments in a PDF using Foxit. Now, when I open it in Adobe
Reader 8.0.0 or 8.1.0 and start scrolling it I get the error message:
"An error exists on this page. [...]" followed by another one stating:
"Wrong number of arguments for a setcolor operator." The uncommented
version does not cause Adobe Reader to exhibit this behavior.

I have attached two screen shots documenting this error in this mail.
I have also attached the commented and uncommented version of the PDF.

Why does this happen and what should I do to avoid this until you
create a fix for this problem?

With kind regards,
...

The answer I got was:

Hello,

We open the files in Foxit Reader,but there is no problem.
If you have any further questions, please feel free to contact us.

Your sincerely,
Kurica
Foxit Software Support
www.foxitsoftware.com

And, so I replied:

As I wrote to you before, the problem was not that Foxit Reader was
unable to show the comments, but that Foxit Reader's comments causes
problems in a PDF-standards compliant reader, namely Adobe Reader 8.

If you understood my problem as stated above and still chose to 
test the document I sent to you only in Foxit Reader, then that must mean
that you are encouraging the whole world to install Foxit Reader in
order to read comments generated in your software? When did "Portable"
in PDF (Portable Document Format) stop meaning exactly that? Thus,
given that PDFs are meant to be portable, I encourage you once again
to look into this problem.

And have not heard anything else from them after that.

Here's two of the screen captures that I sent to them.

First, the error message:


And, here's one of the "invisible" annotations created by Foxit in the same document: