#1449 Cannot read embedded annotations from exported Skim notes

[Will]

Skim is having trouble displaying embedded PDF annotations that are created by exporting Skim notes using "Export… With Embedded Notes".

The highlighted region appears properly, but there is no clickable target to expand the note and view the contents. The annotations appear properly in Preview and Acrobat Reader.

If I right-click on the highlighted text, and select "Remove Note" and then perform "Undo", the clickable region appears, and I can view the note.

Embedded annotations created using Acrobat appear properly in Skim. Only the embedded annotations that are created by exporting Skim notes do not appear properly.

I can use "Convert Notes…" to view the annotations (that works properly), but I would prefer to view the embedded annotations without converting them to Skim format.

Steps to reproduce:

1. Add a note to PDF using the "Note Tool" and enter some note text.
2. Export using "Export…" and then select "With embedded notes".
3. Open the exported file in Skim. There is no way to view the note text (without performing "Convert Notes…")

I am running Skim Version 1.6.8 (138), Mac OS 10.15.7

[Christiaan Hofman]

We don't control how notes are embedded, this is all done by Apple's PDFKit.

[Christiaan Hofman]

I now understand you mean that it does not expand in Skim. That is not a bug, that's just the way it is. Skim does not handle embedded PDF annotations. (This is not even something Apple's makes accessible, they kept it private for Preview).

[Will]

Thanks for the update. Could you clarify a bit more?

If I create a note using either Preview or Acrobat, I can view the embedded annotation in Skim. However, if I create the note using Skim (and then export as embedded annotation), I cannot view the annotation in Skim (but I can view it in the other readers).

Presumably Preview is using PDFKit to create the annotation? Or is it with a private API?

Is there a way to make the embedded annotations that Skim exports behave like the ones created by Preview or Acrobat?

[Christiaan Hofman]

I have no idea. Again, Apple's PDFKit creates the PDF data including the embedded notes. And Apple is not documenting much about what it does. What Preview does is for a part really private, they don't even use proper annotations.

[Will]

OK, thanks for the response.

I suppose the most important thing is that other PDF viewers can view Skim's exported notes, but it is a little strange that Skim itself cannot. As a limitation of PDFKit, we can only hope that Apple will fix it…

[Brecht Machiels]

I can't view the annotations in Preview (version 11.0 (1033.4)) either, other than in the "Highlights and Notes" sidebar, which isn't very convenient compared to the pop-up on mouse hover in Acrobat.

[Peter Michael Gerdes]

So the annotations in PDFKit do let you load them and skim is clearly saving them correctly. However, Skim isn't using PDFKit to load annotations from a pdf file when there is no SkimNotes file. Rather it is using CGPDFDictionary from CoreGraphics and trying to reconstruct the pdf notes that way. While I think that is probably supposed to work, it looks like that method tries to fetch some global annotation dictionary while the way PDFKit saves annotations requires that you loop through each page and fetch them from there. Maybe if I have some time I will try to work out exactly what is going on here.