The pdf files generated open some but not all pdf readers. They read OK in Firefox but not SumatraPDF for me (I have not done extensive testing).
I suspect this is a result of the internal pdf generator being on “Obsolete” feature, so not being actively updated at the moment. The long term solution is discussed in Support for emailing documents in Desktop edition - #6 by Patch
in the interim the work around is get your customers to try another pdf reader or
then email the attachment, which should work with most pdf readers.