I have received word about this issue from GRAPHISOFT developers.
So the issue was that in ARCHICAD no default color was assigned to Bluebeam Space elements, so when they were sent to Bluebeam, they were displayed using Revu's default blue color (which cannot be turned off).
In the latest AC21 build (build 6000 - currently in Preview status), this was changed: Bluebeam Space elements are assigned a transparent color, so they will then be displayed in Revu using this transparent color instead of the default blue.
So, this was actually not an issue with PDFTron, although it is true that these Space elements did not get into the PDF files created using the "Print to PDF" command.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
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2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28