We finally "rotated" the contents of the two layers on each document. The .pdf files of the rotated documents were in the range 370 - 512 Kb. The .pdf files created prior to rotating were between 5.7 and 31.2Mb. They were all emailed and we've had favorable replies, so they didn't cause any trouble. We don't know why it worked, but it did.
Would switching to 12.5 solve this problem in the future?
Thanks for everyone's advice.
Kathryn