Q: iPad USB

You can use it to sync, but not as a storage connection.

USB adapter for iPad

What are we talking about?  Sync cable or Camera connection kit? 

but I need something to transfer documents between iPad and PC.

Camera connection kit.

The usb connector for the ipad as lots of restrictions.

-- It is an input only connection

-- only supports input of pictures & video

-- low power only usb devices.

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You will have better luck with the sync cable.

There are lots of ways of moving files.

A simple and popular way to copy files and share files amoung your devices.

https://www.dropbox.com/

Using iTunes to transfer files:

http://support.apple.com/kb/HT4094?viewlocale=en_US&locale=en_US

Files Connect -- The swiss army knife of remote file connect

https://itunes.apple.com/us/app/files-connect/id404324302?mt=8

iExplorer -- " iExplorer's disk mounting features allow you to use your iPhone, iPod or iPad like a USB flash drive."

http://www.macroplant.com/iexplorer/

Windows File server

http://itunes.apple.com/us/app/filebrowser-access-files-on/id364738545?mt=8

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Airstash

This dual personality device is both a WebDav device which allows you to transfer files via Wifii between device & iPad and a thumb drive which works with compuers.

Compatibility

Wireless operation:

     Web browser WiFi b/g capable device

     Optimized experience for iPhone, iPod touch, and iPad via HTML5

     No internet connection required

USB operation:

     Compatible with most operating systems that support USB Mass Storage Class (thumb drives)

     Firmware update via file drag and drop

http://www.airstash.com/