PDFs are converted to Print Replica (one of several fixed layout KFX formats) for Scribe, but they remain PDF for other Kindles and the Kindle apps. Scribes download PDFs if they were sent prior to November 2022, or if conversion to Print Replica fails.
PDFs are like unwanted step children in Kindle platform: they lack sync services, and even basic reading features like highlighting and dictionary lookup in the Kindle apps. Print Replica has those services and features, but so far only Scribe gets personal documents in this format, so it is moot unless you have more than one Scribe.
I've been waiting for months for Print Replica format personal documents to start showing up on other Kindles or the Kindle apps, with first class status. Side loading the corresponding KFX file to the Kindle apps seems to work with nearly full functionality (except for sync of course). I think the 'sticking point' with doing this now is that Sticky Notes aren't supported.
My guess is that there will be other Notebook clients in the future (Kindle for iPad, Fire Max 11, the all new Oasis 8" with stylus support that will surely launch next month
) and those apps will also be eligible for Print Replica format. (it would be nice if you could at least view scribbles across the entire platform)