Google Docs Viewer, a service that lets you preview online PDFs, Word documents and PowerPoint presentations, is now available for iPhone and Android devices.
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.