WhatsApp, Instagram, plenty of forms and most slide decks will take a picture but not a PDF. Turning pages into images solves it in a few seconds.
An image goes anywhere a photo goes. That is the entire advantage, and it is a real one when a form or an app refuses documents.
The trade is that images cannot be searched or selected, and a dense page of text saved small becomes hard to read. If the recipient needs to read it properly or copy from it, send the PDF.
PNG holds sharper edges on text and line drawings. JPG produces much smaller files and suits pages with photographs on them.
Not yet. Each page is saved separately for now.
Choose a larger size. Print size gives roughly twice the detail of screen size.
No. The pages are drawn by your own browser.