Hi, to add a custom appearance to an annotation, see this forum post.
https://groups.google.com/d/msg/pdfnet-sdk/VKV_M2r8c_8/ELadsgU3R9gJ
The above just adds an image, but you can add any content you like. See the ElementBuilder sample on how to add text and paths.
https://www.pdftron.com/pdfnet/samplecode/ElementBuilderTest.m.html
Note though, that if someone opens up the PDF in a different PDF reader, and resizes the annotation, or in the case of freetext, changes the text, then your custom appearance would be lost. Moving is fine though.
Furthermore, for your own viewer, you will need to hook into the event in tools for resizing your annotation, and trigger your custom drawing, or again it will revert to the default. Again, you don’t need to do anything for moving of the annotation.