I finally solved the problem, at least for myself.
I hadn't experienced much in the way of the *random* crashing that others mentioned (only when I was doing a large import). However, I had previously been unable to save as PDF, and the behavior I experienced was inconsistent. Usually, TT would just vanish (crash) during the save. Sometimes a PDF file (incomplete) would appear before the crash, but usually it wouldn't. Sometimes, TT would display an error message popup before I had a chance to click "save" - while I was selecting what to include in the PDF. It was all over the place.
What fixed it was installing TT fresh on a different PC (a tower instead of a laptop), installing all the updates and rebooting. On this machine, I could save to PDF with no problems. In fact, I was able to do the regular PDF save as well as the "key worksheets" and "all worksheets" PDF save.
I don't know what made the difference on my tower. It's more powerful, and I have fewer applications installed on it (e.g. no Office). Maybe that's it. Maybe it's the fact that my tower (where the save worked) *doesn't* have the Oct 2018 Windows 10 update, which for some reason hasn't been offered yet to this machine. Maybe it's that on my laptop I had opted for TT to not send performance feedback, but I left that option enabled on my tower.
My hunch, however, is that the issue here is a race condition - a timing issue. That's usually the cause when the symptoms are inconsistent, since otherwise the program should behave deterministically. So running on a different machine with different characteristics may have itself been the cure. For example, my laptop has an SSD, whereas my tower has an old-style spinning HD (much slower to write files to). It's entirely possible that the export operation was executing too fast on my laptop, causing the race condition to hit a failure path.
Anyway, it's just a hunch, but it would be good to know if others who hit the "crash when saving to PDF" issue also are using an SSD.