iTunes doesn't allow you to sync iPad to PC back. So if you want to backup files on iPad or sync iPad to PC, you have to use the iPad to computer transfer software which is free to download.

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Tutorials of How to Sync iPad to PC
Apple built iTunes to transfer files from PC to iPod, iPhone, and iPad but in order to
the copyright of the music and movies, it doesn't allow iPad users to sync iPad to PC.
iTunes is just as a unidirectional transfer program. So how to sync iPad to PC
becomes one of the biggest problems. Here we recommend you use iPad to Computer
Transfer software which can be free downloaded and is easy to use.
Tip: For Mac users, please go to iPad to Mac Transfer.
Step by Step Guide
Free download iPad to computer transfer and install it firstly. Then launch it on your PC. The
screen is as follows:

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Step 1: Connect iPad to PC
Link your iPad connector dock to the PC USB cable. Then the software will automatically
recognize your iPad and show the iPad content in the UI.
Step 2: Select
From this left list, you can select the files which you want to sync to PC. For example, if you
want to sync photos, just select the iPad photo list and then check the photo on the right pane
you want to sync.
Step 3: Sync
Click the Export button, then choose Export to Disk Only and start to sync iPad to PC.

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While enjoy the iPad movies, music, photos on PC, don't forget to share this useful tool with
more guys who need to sync iPad to PC!
More Guides:
How to Transfer Videos from iPad to Mac?
How to Transfer iPad Playlist to iTunes?
How to Transfer Music from iPad to Mac?