Sioyek is a PDF viewer designed for reading research papers and technical books.

Has basic scroll, mark, highlight functionality but an interesting 'portal' one. You link one location in the document to another (opened in another window) and the extra window always displays the closest linked location as you read along. Seems useful for consulting e.g. figures or tables while also reading along the main text at the same time.

Can also be extended for OCR, text-to-speech, auto-download and translation which seems good.

Cross-platform