The PDF file format is often used to create documents that are read-only, for instance, ebooks, an announcement etc. The purpose is to prevent the user from editing the document, either intentionally or accidentally. Usually, a PDF viewer does not have any editing tools, which makes the PDF file format the "safest" document format ever.
PDF-XChange Viewer is one such PDF viewer with one useful addition. It allows you to add comments and annotations in the form of sticky notes to the pages of a PDF file. Suppose you want to add some comments or notes to certain pages of a PDF file for future reference, you can do so with PDF-XChange Viewer. The comments can be added at any position on the page and they appear as a small talk-bubble with a sticky note. The sticky note can be closed or opened as required, and the position of the sticky note on the page is shown by the talk bubble. The actual position where the sticky note will pop-up when the page is opened can be anywhere on the page and can be adjusted by dragging the note to a suitable location, for instance, the side of the page so that it doesn't obstruct the text.
The annotated PDF file can then be saved in the PDF format, and can be viewed with the sticky notes intact in any PDF viewer. This is another beauty of the program. So if you send a PDF file with comments attached to your friend, he or she can view it like a normal PDF file in any PDF viewer.
Apart from this handy annotating feature, PDF-XChange Viewer is actually a very good PDF viewer. It's opens fast and loads very large PDF files very quickly. Even pages with high image content scroll very smoothly. An amazing good PDF viewer with a handy utility.
PDF-XChange Viewer is available both as an installable application and a portable application. The portable version is significantly smaller and lighter than the "normal" version. I recommend you use the portable one.
Also read: How to bookmark a page in PDF