Problem displaying pdf files after update to lubuntu 18.04

Opening the file in Adobe Illustrator you'll find out that the glyph font is actually https://en.wikipedia.org/wiki/Symbol_(typeface) It likely comes with e.g. Adobe Reader, but not with evince. At the same time you probably don't have the font installed as system font, so there is nothing to fall back to. Props to envince for not silently substituting it with other font.

Last thing that could make this display correctly is if the font was embedded at creation with the file (full info within the pdf itself). However based on https://root-forum.cern.ch/t/embed-fonts-in-pdf/21666/2 ROOT does not allow to embed fonts to keep the file size small.

tl;dr Make sure the font is set to "Helvetica" which unless on mac will give you Arial, but either way you should be able to display the symbols correctly then.