Thank you for continuing to help me with this!
I had already found that article, and it did not help. The article predates the release of 64-bit Reader and refers to a similar but different problem with 32-bit Reader in 64-bit Windows 10. Nevertheless I reviewed the recommended steps.
In Step 3 I cannot enable PDF Preview Handler as suggested because it's not there.
In Step 4 the registry is already set exactly as described. But this entry is in WOW6432Node and pertains to the 32-bit Reader. The 64-bit Reader has setup the 64-bit registry entry as shown below. I presume that 64-bit Outlook would find this and work correctly.