Some mathematical symbols are not rendered at all, not even showing an empty rectangle or something similar. For example, the following document: http://www.jmlr.org/papers/volume7/belkin06a/belkin06a.pdf in page 7 contains a delta symbol (traingle pointing upwards), which is rendered correctly in Adobe's PDF reader or chrome's built-in PDF reader, but on not software based on Poppler (such as Okular).
delta shown perfectly for me, i guess this is another case of wrong fontconfig substitution configuration, nothing we can do to fix your system. For reference this is the font my system uses tsdgeos@xps:~:$ pdffonts -subst http://www.jmlr.org/papers/volume7/belkin06a/belkin06a.pdf | grep Symbol Symbol 79 0 Standard Symbols PS /usr/share/fonts/gsfonts/StandardSymbolsPS.otf Symbol 74 0 Standard Symbols PS /usr/share/fonts/gsfonts/StandardSymbolsPS.otf Symbol 97 0 Standard Symbols PS /usr/share/fonts/gsfonts/StandardSymbolsPS.otf Symbol 201 0 Standard Symbols PS /usr/share/fonts/gsfonts/StandardSymbolsPS.otf Symbol 273 0 Standard Symbols PS /usr/share/fonts/gsfonts/StandardSymbolsPS.otf
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