Currently, it’s impossible (or very difficult) to edit a PDF or image in Obsidian mobile (see related feature requests below), because we can’t access the original file, and open it (and not a copy as on iPad) in the app we want.
A quick workaround would be to add an option to reveal the file (image, pdf, etc…) in the system explorer : thus we could quickly access the original file in our favorite PDF reader / image editor, or use iPad’s markup option (or Android equivalent) to directly annotate.
Of course it would be better if we could access the “Markup” option or “Open with…” directly from Obsidian, but “Reveal file in system explorer” is maybe easier to implement as a workaround ?