Great job! I can see this being very useful, at the very least in the academia. For example, in meta-analytic reviews, you often go through hundreds of papers, identifying, highlighting and annotating the relevant hypothesis test details, among many irrelevant weeds. Tools that streamline this process are very handy, so that you can easily come back later and grok your past work. Pen and paper works for an individual, but can drown you in piles of binders, and is less easy to collaborate on/share.

Some quick, very minor UI comments: on a large screen, workspace switching could be available immediately through single-click icons instead of a dropdown menu. Similarly, minimizing open stickers could be done with a dedicated icon instead of through the “…” menu (right-click suggested in an example in the quick start guide didn’t work for me). Otherwise everything feels pretty intuitive.

One thing I’d want to learn from the introductory guides is if adding the metadata (comments, notes, highlights, etc.) changes the original PDF file (always, optionally, never?), so that for example you’d be able to send an annotated document right away, or need to keep backups of unmodified originals, etc. (I strongly assume the metadata is stored separately, as PDFs can be unwieldy to modify, but you never know.) In general, exporting/printing notes seems like a natural feature. I like that it’s easy to export clips.

As a casual right now, I don’t have any input regarding the possible business model, but will certainly be interested in where you go with it.