Arthur - thanks for your thorough and helpful suggestions, but:
1. 250 GB available on my 500 GB flash drive
2. 16 GB 1600 MHz DDR3 RAM
3. Many - I am always running multiple apps on multiple screens - have been for years
4. Nothing unusual here - other apps, which require more processor power, run fine.
5-6. I think I would only try these time consuming tasks if I was having problems across apps - not that they are bad suggestions.
7. No, I have never used any of that.
8. Similar feelings as to comments on 5-6; did repair permissions (not using recovery partition) but they were minor fixes. Disk verified fine.
I'm running a 15" MacBook Pro Retina 2.7 GHz i7 with 16 GB RAM and a SSD, which is less than a year old so I don't think it is a power issue or a disk issue.
Sorry, I shoud mention that this doesn't happen all the time; it is random and I haven't found a pattern yet. The same PDF can hang one time and later open fine.