Created attachment 120969 [details]
File with some objects from Formula Editor
Signs as the equals sign and the plus sign appear as other special characters (a floppy disk and an envelope, respectively).
Try to download and install the font from http://cgit.freedesktop.org/libreoffice/core/plain/extras/source/truetype/symbol/opens___.ttf?h=libreoffice-5-0-3
Please let me know if this won't solve the issue.
*** This bug has been marked as a duplicate of bug 76239 ***
It works. Thank you so much.