The issue is that the document specifies a font (here: Calibri), but the font is not embedded in the PDF. If the client computer also doesn’t have that font installed, then it is up the to renderer to make the best of it. And each PDF viewer may have its own generic solution of substitution fonts, that may look good in some instances, but not in other.
If the readers of the PDF have the used fonts installed, then this discrepancy wouldn’t happen.
In order to have a PDF look (almost) the same on all platforms and programs, the maker of the PDF should embed the used fonts, if they can (not all PDF makers have provisions for that, and it does make PDFs larger).