How to sync an iBooks collection created on my iPad?
follow the instructions given on this page:

https://support.apple.com/en-gb/HT201478

Find the section "Organise your library" then scroll through to the sub-category called "Create, view, and organize collections."

The following is the part that will sort it out:

"To automatically sync your Collections with your other devices, sign in to all of your devices with the same Apple ID. Then, on your iPhone, iPad, or iPod touch, go to Settings > iBooks and turn on Sync Collections."

If in future you wanted to reverse this and sync all of your collections FROM your Mac to your iPad, this is what you'd do:

"To sync from your Mac, choose iBooks > Preferences from the menu bar at the top of your computer screen. Click the General tab, then select "Sync bookmarks, highlights, and collections across devices." "

NOTE - if you have the "Sync Collections" option turned on on your iPad AND on your Mac at the same time IT WILL NOT WORK. Choose one or the other.

Hope this solves it for you!