I'm trying to add annotations/highlights to a PDF using the Document Viewer (Evince) in Ubuntu 16.04. When I add a text annotation, the annotation box comes up for me to type in but when I close it there's no way to re-open it. It's in the annotations pane but I can't see it (or the icon for it, which is supposed to be there) in the document. Highlight annotations don't work at all.
Is it just that the Document Viewer doesn't support annotations/highlights for scanned documents?
If so, and the feature hasn't been added in 3.20 or is planned to be added in 3.22, then tell me and I will file a bug report for the misleading appearance of the annotation box/the failure to grey out the options for scanned documents.
Document Viewer (evince) supports adding and saving annotations in scanned documents. Evince 3.18.2 from the Ubuntu 16.04 default repositories also supports highlighting of text snippets, but not of images. The instructions for adding annotations to a PDF document in Document Viewer are here. If these instructions don't work on your system, then report it as a bug.
As soon as you close the little annotation window, the icon for the annotation should appear in the same place in the document where you added it. The annotation icon looks like a tiny post-it note which is attached to the document by a push pin. Clicking on the note shows the annotation in an annotation popup window.
Highlight annotations require a text to be highlighted. As such, they generally don't work on scanned documents as these do not include a text layer.
If the document however contains a text layer (for example, after using OCR software) highlight annotations can be applied with Evince.
Text annotations should work though. Evince supports only notes, as far as I can see. To reopen a note double clicking on the icon should work (and does on my Ubuntu 16.04, Evince - Document Viewer 3.18.2).