Firefox

Because you don't always want to permanently save to the drive the PDFs you find on the internet, often you just need to skim them and that's it, just like a webpage

It keeps being downloaded as a regular download to the default download folder, filling up disk space and the file needs to be deleted by explicit command. With the change it's a temporary download and is erased when cache is purged.

PDFs are particularly annoying like that. On one hand most browsers offer to open them in the browser, but on the other hand it's a "file" rather than a document. Imagine if your browser decided to download all the HTML webpages that you visit to your download folder, instead of the temporary cache. That's how PDFs are handled.

To make it more complicated, sometimes you'll want that and other times you don't.

It's also really annoying in terms of GDPR compliance, because some users gather a lot of PDFs in their download folder without even realising it. I empty my download folder daily, just like the trashcan, just to be sure that I'm not keeping things that I'm not supposed to keep. This is only/mainly due to the browsers handling of the PDF format.