For a long time the best lightweight PDF reader for XP is SumatraPDF 3.1.2 which based on MuPDF. There is alternative such poppler/xpdf and ghostscript, all of them use rather unfriendly GPL-like license. PDFium which used within Chromium is new BSD license or APL 2.0 but has crude packaging (typical of google).
I stumbled on https://github.com/vfr/UXReader-Windows a nice pure Win32 API UI library for pdfium, then made little quick change on UXReaderMainWindow.cpp: commenting ChangeWindowMessageFilterEx (which break XP) and add simple argument passing using CommandLineToArgvW so it now will open file from association / cmdline. Note that it's just a barebone demo for the library but IMHO suck less than FoxIt reader. I mean whats the point having a light renderer if you bloat everything else.
Campared to MuPDF I think it's faster (and smaller too), still when rendering image-rich page with multiple overlays and transparency nothing beats Adobe Reader!
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download: UXReader.7z (1.6 MB) require VC++ runtime 2017