http://docs.google.com/?action=updoc&formsubmitted=true&uploadURL=DOCUMENTURL
This works for documents (.doc/.txt/.html/.rtf/.odt), spreadsheets (.xls/.csv/.ods), presentations (.ppt) and PDF files.
In Windows, you can easily create a batch file that automates the upload of multiple files to Google Docs, assuming that the browser is open and you are already logged in to a Google Account. For example, the following text can be copied in Notepad and saved as a .bat file. After executing the .bat, the two PDF files will upload to Google Docs in separate Firefox tabs.
start /d "%PROGRAMFILES%\Mozilla Firefox" firefox "http://docs.google.com/?action=updoc&formsubmitted=true&uploadURL=http://www.fireworksafety.com/pdfs/FireworkSafetyTest.pdf"
start /d "%PROGRAMFILES%\Mozilla Firefox" firefox "http://docs.google.com/?action=updoc&formsubmitted=true&uploadURL=www.fs.fed.us/fire/safety/wct/2002/brochure_2002.pdf"
start /d "%PROGRAMFILES%\Mozilla Firefox" firefox "http://docs.google.com/?action=updoc&formsubmitted=true&uploadURL=www.fs.fed.us/fire/safety/wct/2002/brochure_2002.pdf"
A similar option is available in Google Toolbar for Firefox, which lets you open documents from the web in Google Docs.