Hey everyone, So happens to be my first post on this forum..
I also found a seemingly random full sized PDF/saved site page in my downloads folder. I was very confused on why it was there..
However; i managed to reproduce the bug multiple times in a row.
On the iphone do the following to reproduce the bug:
Open Safari (it only seems to happen in Safari).. i did not test other browsers.
1: Make a screenshot with the physical buttons (something people do on accident all the time).
2: Open the thumbnail pop up that appears in the lower left corner of the screen
3: Swipe up and close the current screen
4: The thumbnail will re-appear in the lower left corner.. do not touch it and let it disappear on it's own.
And voila; you got a full sized page saved...
I can imagine allot of people do this on accident. They make a screenshot by accident and click/touch the thumbnail. Then they want to get away from that page and swipe up.. and that saves the PDF.