Here's what I've discovered: Outlook has two "versions," one for iCloud and one that is an Outlook data file ("My calendar"). You can check a box in Outlook to enable either of them (go to the Home tab and expand the navigation pane). The iPad does not sync with the Outlook data file. Anything entered there has to be manually copied to the iCloud version of Outlook in order to sync with iPad. So, to sync with iPad you have to uncheck "My calendar" in Outlook and then check "iCloud" and use that version of Outlook. But when I use the iCloud version, Outlook will not accept appointments sent to me by clients. It also screws up my communication with clients in other ways. So using the iCloud version of Outlook does not work for me.
I've discovered a third-party software that seems to solve this issue: Code Two Sync for iPad. You can Google it for reviews, the download link, etc. I downloaded and installed it and it immediately copied (synced?) all the appointments in the Outlook data file version of Outlook ("My calendar") to the iCloud version and synced them with my iPad. There were a bunch of duplicates, but I think there's some way to fix that. However...it also crashed Outlook on my computer (I have an older computer that is having issues). So I needed to uninstall it. I repeat, this is because my computer has been having other issues, I don't think it is the software's fault. I may try it again when I have more time to troubleshoot. Anyway, you might try this software and see if it works for you.