First of all, let me say that okular is a fantastic pdf reader. In some pdfs, greek letters do not show up in the document, although it does in other viewers. I tested in the MuPDF and in the chrome pdf viewer, and the letters appear normally there. Here is a link for a pdf with the precise example: http://www.jmlr.org/papers/volume12/hannah11a/hannah11a.pdf At page 1929, for instance, in the line where you see $X_{ij} |θi,x ∼ N(μ_{ij} , σ^2)$, the $mu_{ij}$ just do not appear in okular.
Do you have fonts-texgyre installed? Could you please try to remove them?
I do have them installed. Is there any way to solve it without removing the fonts?
Please try at least to see if removing them solves the issue. It was the solution for me, and I remember other bugs related to those fonts: https://bugs.kde.org/show_bug.cgi?id=357822 https://bugs.kde.org/show_bug.cgi?id=335058 https://bugs.kde.org/show_bug.cgi?id=332595 So I'm not sure if there is a real fix, if not playing with the fontconfig configuration so that font is not used. But if it's not used, removing it shouldn't be a problem. I can see those characters using gsfonts.
Yes, by deactivating them it works. Thanks !
There's nothing we can do here, at most it would be a poppler bug. What poppler does is you don't have the font, asks fontconfig for a substitute font and tries to use it as best as it can. Either the font fontconfig returns is not a good substitute or poppler is misusing it, anyway it wouldn't be an Okular bug.