I'm using groupdocs annotation 18.104.22.168. But there are some issiues. I make a two gifs about them. You can see them as attachment. I tried those issues with 22.214.171.124 but they're not changed. Also i'm tested them with your front end mvc showcase.
When i try to use polyline, pointer or distance tool they are not showing correctly in web but after export with comments, they are shown in pdf as it should be. For example if you are using resource redaction tool, when you start selection it shows a temporary rectangle to show you which area you are selecting. But in these three tool it doesnt. Another issue is after selection operation complete, viewer doesnt show object. For example if i use pointer tool, after selection operation it shows only rectangle and it doesnt show pointer inside rectangle. If i export document with comments, pointer is looking well in pdf document. You can see this in "pointer.gif".
Other issue is, when i try to use text annotation, strikeout, text replacement,text redaction or underline tool after export with comments. Acrobat Reader gives error for pdf. It give "Out of range" error. You can check it from "text-strikeout.gif"
Also in watermark tool we cant change font size bigger then 17. Can we change it and can we add custom colors? And can you add a position property like viewer (Diagonal, TopLeft, etc.)
Thanks in advice,