Is there a free pdf viewer with the following properties?
-It can annotate.
-Annotation will be preserved when the document is sent by gmail.
-Color inversion (Evince has it) or color setting.
I've done some research in this area some time ago - from the free PDF viewers I was able to find, Okular may be the only one which supports annotations, but they're stored separately from the file, so they're not "gmail-sendable".
It also does support color inversion.
The reason for the lack of "embeddable" annotations seems to be that the underlying library for working with PDF files (poppler) does not support writing annotations into a PDF document yet.