The PKGBUILD exists: https://github.com/msys2/MINGW-packages/tree/master/mingw-w64-pdfium-git but it builds from git sources, so we need to use makepkg to get it installed. Then we should update it fairly regularly with bug fixes.
Added a message in r23683.
Original ticket: #1230
We now use pdfium-binaries : ticket:2470#comment:12
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2401