+1 vote for save to PDF.
Being able to share and archive reports outside of TestRail is mission critical to ensure our stakeholders have insight into testing. Having to use the ZIP/HTML option is not end user friendly or intuitive for users who aren't expecting it. If there is any confusion with the file, it's likely that it simply won't be read.
While printing to a PDF is a usable workaround, it turns off a lot of users who expect the direct "save to PDF" functionality to be present. When they find it is not a default option, it damages their faith in the overall quality of the tool.
Anything that can help make the reports as easy to generate and consume as possible is a definite win for the tool.