My coworker sent me a paper that we are both working on as a pdf and I need to use LaTeX to continue to work on it. Can I open the pdf in LaTeX and edit it and add to it?
No. This is not possible. Your coworker needs to send you the
.tex file which contains the source for the PDF.
If you only have to add to the pdf file then the pdfpages package could help. But if you have to change anything on the pages your co-author has written then you're out of luck and your co-author will have to send you the latex file.