Can I backup my iPad in Dropbox?

No, it is designed to do it to iCloud or to a computer as a single backup package, not individual files the way you could when backing up a computer.

To learn "How to back up your iPhone, iPad, and iPod touch" click here --> https://support.apple.com/HT203977

If you wish, you CAN, in addition, backup yiur individual files within apps to any other ancilliary “cloud” data storage/backup services, like Google Drive/Docs/Photos, DropBox, BOX, Amazon cloud storage services and other similar data storage/backup services.

I backup lots of different individual files to Google Drive/Photos and DropBox, as well as Apple iCloud.

You just cannot do a complete, whole iOS device backup to these types of services.

You can have multiple backups of individual app file data, though, stored elsewhere.

Just to clarify things.