Where can I download iDoceo 8 for Mac?
From the Mac App Store or following this link
Link for Mac App Store iDoceo's page
Link for Mac App Store iDoceo's page
If I have bought the version for iPad or iPhone, do I need to pay again for the Mac version? (or the other way around)
No. It is an universal purchase, meaning if you buy iDoceo for any device iPad, iPhone or Mac, you buy iDoceo for all of them, so it is free to download it. You are welcome.
Then, why do I see the purchase button 'Buy' in the Mac App Store?
This is due to an Apple error on their App Store. It means that for the Mac App Store it is not known what purchase you have made in the iOS App Store. If you bought the iPad or iPhone version with the same Apple ID that you are using in your Mac, iDoceo download will be free and Apple will not charge you anything for it.
How can I make sure that I am using the same Apple ID with which I made the purchase in my iPad or iPhone?
If you open the Mac App Store and click in your account, you will see what apps you have purchased (including the ones bought in your iPad or iPhone) and how many of them are available for Mac.
Do I need to have iDoceo 7 or higher in all my devices to be able to synchronise?
Do iDoceo's Backups and restoring them have anything to do with synchronisation?
No. Synchronisation have nothing to do with backups or the method you choose to make or save them.
Synchronisation is automatic, in real time and works exclusively through iCloud/CloudKit.
Synchronisation is automatic, in real time and works exclusively through iCloud/CloudKit.
I do not see the synchronisation between devices option in my iPad
That means you are using an ol version of iDoceo. Please, update to iDoceo 8 following this link.