I have a PDF of a Ph.D. thesis in mechanical engineering, so there are lots of equations. No PDF app that I've tried on OS X (Preview.app,Adobe Reader, Skim, PDFPen) can show the equations properly; some symbols are missing (particularly integral signs). However, when I open the file in Windows (with PDF X-Change) the equations all appear right.
I don't know how the file was generated. Any idea of the cause of this issue?