User has set Open Sans as the font for one of their invoice templates via Global Settings > Invoice Templates. On previewing the invoice using this template and exporting it to PDF, the font displayed is not Open Sans.
As of now, the PDF Export feature does not support all fonts, including Open Sans. So if you have set the invoice template font as Open Sans and export the invoice as PDF, the font gets converted to the nearest supported font (Microsoft Sans Serif in this case). If you really need to use Open Sans, you do have a workaround: export the invoice as a Word document, make the changes to the font there and then save it as a PDF.