I’m also currently figuring out how to deal with PDFs in Obsidian. So far the best way forward seems to just embed a PDF into a markdown file via “![[Name of PDF]]” (without quotation marks). The Markdown file is than taggable, you can code and do whatever you usually expect from a file in Obsidian.
Not sure if that helps in your case, you seem to want something that is specifically for annotation. What would you expect to be the use-case of doing that in obsidian instead of importing the annotated PDF?