As long as you have not downloaded the PDF you can cancel. Once you have downloaded you can no longer. This because once issued as a PDF a train ticket can not be un-issued. A train ticket is a proof of payment. Since you cannot give the proof of payment back you cannot get your payment back either.
ÖBB wants to avoid people buying tickets, downloading the PDF, then applying the refund, and then using the PDF to travel anyway...
ÖBB for that reason does even tell you to not download your ticket until shortly before departure.
Of course, ÖBB could maintain a separate database of still valid tickets, and some railways actually do that. However synching data between databases of 20+ railways is not a trivial task. Some railways solve the issue by just not selling international tickets except for a handful of destinations (eg. France, Spain). ÖBB apparently has chosen to solve the issue differently.