Is anyone observing that PDFs generated by Tessitura (v. 15.1.3) are not readable on newer iPhones or iPads?
Yes, This was an incompatibility between the old PDF creation software in Tessirua / infomaker that was introduced in on iPhones & iPads when IOS 13 was released. Apple introduced a version of the Preview software that produced lots of dots rather than text.
There is a post from Patrick Schley (Past Staff Member) somewhere around the site about this issue.
The suggestion is to load the Acrobat Reader app on the device and open the files with that software rather than the iOS 13 built-in Preview software. This works OK for me but is a number of extra steps.