TL;DR: Annotation Studio does now support import of text content from PDFs that contain a text layer. There are a large number of documents that people have in PDF format, for which Annotation Studio will work well. There are an equal or larger number of PDFs that won’t.
This doesn’t mean it supports PDF formatting or images, but that if your PDF started out life as a Word Document or a text file (and was subsequently converted to a PDF), it should be possible to import the content of that file (minus the formatting).
If your document started out life as a piece of paper, and was scanned, it probably won’t work, unless the scan was of a high quality, the text was formatted simply and was extremely clear, AND the document was OCR’ed (was run through an Optical Character Recognition process) after scanning. If all three of those criteria are met, there’s a chance of your content being imported successfully.
See this post for more details.