This trick is almost impossible to figure out without someone telling you how to do, but it isn't difficult when you have seen it once.
Let's assume you want to make a pdf of a web page.
- Tap the share button top left and choose print.
- Pinch out on the thumbnail image of the web page. This opens a preview of your pdf.
- Tap the share button in the top right corner and do whatever you like with it.