Submitting an official transcript vs unofficial transcript for postgraduate studies; why the difference? What is the reason that some post graduate schools only require an unofficial transcript upfront vs. an official transcript(referring to online application submission). And if they require an unofficial transcript, but you have an official in pdf form, is that fine as well.

Have schools come to the conclusion that an unofficial transcript is most likely valid, have there been less incidents involved in tampering with original documents? Is this related to how less schools are requiring standardized testing?
The primary issue is expediency. It can take time to issue official transcripts. The sooner a candidate has a complete application on file, the sooner it can be evaluated. In addition, most schools that may allow unofficial transcripts generally require an official transcript before matriculation. If it is found that the unofficial transcript doesn't match the official transcript, then that would be grounds to cancel the offer of admission.