As Mark H would have said to me "Read The Manual" (except that is not quite what he would have said!).

Using the -xpp-pdf-annotation: "type={note} text={hello world}"; does the trick fantastically (and will also output the contents of an attribute instead of set text - I am assuming this is not possible in item formats though!

[edited version as per Jonathan's remarks]

Indeed creating notes is simple.

And if you are using classic IF/XMLmode, remember that you can add a CSS stylesheet that contains the rules with the pdf css properties, like the -xpp-pdf-annotion one. When you name that CSS file according to the style you use for that job, you just add the -pdfba option when printing. Direct PDF will use that CSS file and produce a PDF with annotations

In the PDF Support in XPP manual you find the following on page 8-2 [PDF Tagging Features]:

CSS is used to tag the textual content and generate bookmarks/annotations
(links). Output options allow for creating a fully PDF/UA-compliant
document or using CSS to generate bookmarks and annotations only,
without having to tag all the content.
These properties work with both CSS-XML and XML Divisions, provided
that the XML is well-formed and the markup is based on the logical reading
order of the document.

PS: Mark H. was right, he was always right (even when he was wrong) 

Bart Terryn said:

And if you are using classic mode, remember that you can add a CSS stylesheet that contains the rules with the pdf css properties, like the -xpp-pdf-annotion one. When you name that CSS file according to the style you use for that job, you don't need to do anything different when printing. Direct PDF will use that CSS file and produce a PDF with annotations

Bart said, "classic mode", but technically it is XML mode (and of course CSS-XML mode) that you can do this as evidenced by what it says in the snippet from the XPP manual.

Also, when using divpdf in such an XML mode scenario you would need to include the -pdfba option in order to get the bookmarks & annotations in the CSS file to be processed.

Bart Terryn said:

And if you are using classic mode, remember that you can add a CSS stylesheet that contains the rules with the pdf css properties, like the -xpp-pdf-annotion one. When you name that CSS file according to the style you use for that job, you don't need to do anything different when printing. Direct PDF will use that CSS file and produce a PDF with annotations

Bart said, "classic mode", but technically it is XML mode (and of course CSS-XML mode) that you can do this as evidenced by what it says in the snippet from the XPP manual.

Also, when using divpdf in such an XML mode scenario you would need to include the -pdfba option in order to get the bookmarks & annotations in the CSS file to be processed.

Thanks Jonathan,

I have corrected my original answer according to your remarks, so that all information can be found in one place.