Text annotations which are directly visible on the document are indeed preferable to those hidden behind a popup in many cases.

I looked into this a bit. As politza mentions here (and is confirmed here), the Poppler glib API, which pdf-tools uses, simply doesn't provide a function to add free-text annotations (it can read and interact with them though). Okular does have support for this, but through Popplers qt5 API.

There is an 8-year-old open issue at Poppler where someone submitted a patch which adds support for creating free-text annotations, but apparently it has never been merged! The patch is not very long, and a Poppler maintainer has indicated that if it were updated and put in merge request form it would be merged.

This means that, in order to get free-text annotations in pdf-tools, the following three things have to be done:

The merge request to poppler glib has to be updated and merged (in C++). Poppler would then provide a function poppler_annot_free_text_new and some other functions related to free text annotations.
Some code has to be added in the annotation_new function of server/epdfinfo.c which calls poppler_annot_free_text_new in a suitable manner (in C).
cmd_addannot and pdf-info-addannot should then work, so one just needs to slightly modify pdf-annot-add-annotation to add a freetext type and create a command pdf-annot-add-freetext-annotation which calls it with that type as argument (in elisp).

I could do (3) easily. Potentially also (2). So if someone with a working knowledge of C++/C were to do (1) and preferably also (2), we'd have free-text annotations!

Alternatives:

Adding free-text annotations via Popplers qt5 API. I don't know how feasible this is.
Adding them by switching to MuPDF (or adding it as an additional backend). This is implemented (and works, although still buggy and with a lot of overhead) here and here.

Request for: immediately visible ``free-text'' annotations #82

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