PST files are tricky because they often get to hundreds of MB or GBs in size. And every time Onedrive detects a change it will create a backup of that file and

PST files are tricky because they often get to hundreds of MB or GBs in size. And every time Onedrive detects a change it will create a backup of that file and sync up the changes. So you quickly end up with hundreds of backup copies of your huge PST file consuming all your Onedrive storage. So the recommendation is to not store your PST files in your Onedrive folder. I assume the same advice would apply if using Dropbox or other cloud storage providers.