Text, shapes, images and signatures are layered on the page. The file never leaves this tab.
Editor questions
Does the editor change the existing text?
No. Edits are layered on top of the original page, and nothing already in the document is rewritten. To replace a line, draw a filled shape in the page colour over it and type the replacement on top.
Where does the file go when I export?
Nowhere but your download folder. The document is assembled in this tab with the same engine the rest of Ream uses; there is no upload request.
Why does my text not match the original font?
Standard PDF fonts are Helvetica, Times and Courier. Matching an unknown embedded font would mean embedding it, which most PDFs forbid by licence. Use Edit PDF to cover and replace instead.