For those of us who plan to use the reMarkable to read and review PDF documents, it is interesting to see that reMarkable's CTO Martin Sandsmark made a
number of contributions to the document viewer
Okular in 2016, including adding pinch to zoom on touch screens.
Perhaps this is the basis for the PDF viewer on reMarkable tablet? If so, that is good news, since Okular supports a number of other backends, such as djVu, ePub, chm and XPS. It is then conceivable that document support can be easily extended to other formats in the future.
The standard PDF backend in Okular is Poppler, but people have also tried dropping in mupdf, with good performance gains. This is the same PDF backend that is used in KOReader.