The problem is that many new IA pdf files have a poor resolution / too high compression from beginning, so their quality can't be improved.
IA viewer doesnìt use pdf or djvu file, it uses jpg images coming from jp2 images; this explains why images seen by the viewer are so beautiful, while pdf or djvu files are poor.
@Sam: About uploading djvu into IA item lacking of it: no, nobody but the original contributor or a sysop can upload files into an item. But it can be uploaded as a new item linked with the original one; its link could be shown into source item adding a comment (a "review"),