Hi exachillus,
There is already an online PDF reader for comics. For books and other non-image documents, the PDF reader has been added in Ubooquity 1.5.0, which I have just released (see next post

).
As for epubs, this is not planned. Epub files are usually much smaller than comics and PDF files, so it's often much easier to download the complete book and read it with a dedicated application.