.ooutliner file format does not work on my iPad

I use OmniPresence to sync my .ooutliner files between my Mac and my iPhone but the sync do not work on my iPad.

Could this be because my iPad is older, running iOS 9.3.5? If so, will there be a fix? Or is there a workaround?

Thank you.

Your older iPad may not read .ooutliner files if you have not been able to update OmniOutliner an a device due to iOS version requirements. Since the latest OmniOutliner on iOS requires iOS 10 or higher, the best solution is to open the document either on your Mac or iPhone, and export as oo3 (OmniOutliner 3) format so that the iPad will still be compatible to sync with your other devices. The newer versions of OmniOutliner still support the legacy file formats.