Q: how do I transfer a PDF from the iPad to a windows7 labtop using a cable?

Unless you're syncing to that computer, you can't.  You need to e-mail it

One work around that might work, open the PDF and take a screen cap of it (hold down the sleep and home keys and release them, like you would a camera shutter) This will take a photo of your PDF and put it in the camera roll.

Then connect your iPad to a computer. Use window's explorer to click on the name of your iPad, and navigate to the DCIM folder. You'll see a ton of folders in there, and somewhere in one of them will be the screen cap you just took.