Thanks for that advice! I was using the browser and now I am using the desktop app and within that app the Add File... command under the tab that carries the label Add Attachment. Now the upload limit seems to be about 300 files per batch and I need to wait some as yet undetermined time between batches. Am I doing it right?

Thanks for the quick response. So I use the Add File command as described. In the window that pops up when I do Add File I highlight the files that I want to upload. I found that if I highlight more than about 300 files then nothing happens, but (in the initial experience) when I highlighted fewer than 300 then it immediately starts uploading and after the upload it starts acquiring the metadata. I let it complete and I try to upload the next batch same way. But now nothing happens even at smaller batch size. I interpret that (without much confidence) as having to wait a bit before the system will allow further requests from my side.

My apologies, I think something else is going on. Interspersed among my 12K pdf files there are also a handful of directories. (Linux here.) If a directory is in the highlighted list then the interaction just hangs without response. Probably I just need to circumvent the directories when highlighting.