I am using a preview release of RStudio (1.2.907) on Debian 9. When I use the help pane to navigate to some PDF vignette and click on it, the PDF file is not opened in the PDF viewer. Instead, I am offered a download dialogue.
Files are opened in the PDF viewer when I use vignette(...) or click on a PDF file in the file pane. However, in both cases the PDF viewer is opened in the background, which is slightly inconvenient.
Got it! Thanks for taking the time to make a reproducible example. This change is behavior is unintended; we'll make sure to get this fixed up in time for the v1.2 release -- we've now filed this at https://github.com/rstudio/rstudio/issues/3443.