Hello. It can be really by font used. Did you try use FreeSans?
Unfortunatelly, this is problem with TCPDF library we are using to generate PDF. There were several reports for this problem to it's creator (for example
sourceforge.net/projects/tcpdf/forums/fo...435311/topic/5192256), but seems like PDF comply all standards and problem is on side of viewers. We know that is no excuse.
Seems like it is used font's problem; especially when using unicode fonts, non-stanadrd characters, etc. Sometimes it was currecy symbol which caused the problem (see Currency Unit Replacement setting).
We will add config option to turn off font subsetting to next release, which can maybe solve the problem.
(You can try it on your own by editing file administrator/components/com_vminvoice/helpers/invoicetcpdf.php and putting "$this->setFontSubsetting(false);" around line 104)
We are also updating TCPDF library to newest release on almost every update, but so far, no fix.