International Postdocs should refer to the International Students and Scholars Office (ISSO) for all visa, immigration issues, and tax treaty information, as well as for guidance on how to get a social security card. Because much of the pertinent information changes often, the ISSO is your best source for information about these issues.
Postdoctoral fellows and paid-directs may want to view the a PowerPoint presentation from a workshop entitled "Tax Issues Associated with Reporting Fellowships" presented at UCSF in February 2018.