The preferred link to USGS publications should be formatted like this:
IAMSLIC News & Events has a bit more info.
The U.S. Geological Survey's Publications Warehouse (pubs.usgs.gov) will complete a process to migrate all of its' on-line publications into Portable Document Format (PDF) files by September1, 2011. At that time, the USGS will no longer support the previous DJVU format for its on-line publications. Libraries and Web site managers shouldlink to the publications citation page for USGS publications. At sometime after September 1, 2011, direct links to DJVU files will stop working and there will be no automatic redirect to the PDF version of those materials.