Google added some features that make the service more useful on a touchscreen device: pinch to zoom (only for iPhone and iPad), buttons for zoom and pagination.
Unfortunately for Google, iPhone's built-in PDF reader and Microsoft Office viewer are much better than Google Docs Viewer because they don't convert documents to images. Some Android phones include a document viewer based on Quickoffice, which also does a better job than Google Docs Viewer. There's also Adobe Reader for Android, probably the best Android PDF reader you can download for free.
