After looking into several annotation tools, UBIAI stood out for its unique support of PDF annotation. No preprocessing is necessary, UBI have an OCR integration so everything is done right there on their site. They have a fantastic setup for collaborations and multiple annotators on one project, as well as a great validation process. They incorporate AI into many of their processes, allowing us to create a complete pipeline on their site, including testing our annotations on real life models. Their site is user friendly and easy to navigate, which provides for a great user experience. We also appreciate how UBI tries to accommodate our requests and incorporate our suggestions into their product.

It's a paid product but worthwhile for the service!