This problem was originally reported to the xpdf project, but they said it was a poppler problem and should be reported to you.  So...

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xpdf 3.04 emits a huge number of warnings every time.  At least when 
displaying PDF files generated by groff.

I'm using xpdf 3.04, groff 1.22.3, on Ubuntu 17.10, with the following installed:
ibpoppler-cpp0v5:amd64 0.57.0-2ubuntu4.2 amd64
libpoppler-glib8:amd64  0.57.0-2ubuntu4.2 amd64
libpoppler-qt4-4:amd64  0.57.0-2ubuntu4.2 amd64
libpoppler-qt5-1:amd64  0.57.0-2ubuntu4.2 amd64
libpoppler68:amd64      0.57.0-2ubuntu4.2 amd64

STEPS TO REPRODUCE:

(Note: I used "env -i" to avoid any influences from my local environment 
variables.  The same problem occurs if the tools are invoked directly)

$ echo "Hello"  | env -i groff -Tpdf > /tmp/test.pdf
$ env -i DISPLAY=$DISPLAY xpdf /tmp/test.pdf
(opens X window displaying seemingly correctly)
Config Error: Unknown config file command 'unicodeMap' 
(/usr/share/xpdf/xpdfrc-arabic:2)
Config Error: Some work needs to be done to suppport this option in the 
Poppler version of xpdf
Config Error: Unknown config file command 'cidToUnicode' 
(/usr/share/xpdf/xpdfrc-chinese-simplified:2)
Config Error: Some work needs to be done to suppport this option in the 
Poppler version of xpdf
Config Error: Unknown config file command 'unicodeMap' 
(/usr/share/xpdf/xpdfrc-chinese-simplified:3)
Config Error: Some work needs to be done to suppport this option in the 
Poppler version of xpdf
Config Error: Unknown config file command 'unicodeMap' 
(/usr/share/xpdf/xpdfrc-chinese-simplified:4)
Config Error: Some work needs to be done to suppport this option in the 
Poppler version of xpdf
Config Error: Unknown config file command 'unicodeMap' 
(/usr/share/xpdf/xpdfrc-chinese-simplified:5)
Config Error: Some work needs to be done to suppport this option in the 
Poppler version of xpdf
Config Error: Unknown config file command 'cMapDir' 
(/usr/share/xpdf/xpdfrc-chinese-simplified:6)
...[many more like this]...