Hi there, Diane Ward!

I understand that you need to re-sync some PDF files from your computer to your iPad. You can do this a few ways. Here is a link form the iOS 9 help page that should provide some guidance:

Read PDFs

Sync a PDF. On a Mac, add the PDF to iBooks for OS X, open iTunes, select the PDF, then sync. In iTunes on your Windows computer, choose File > Add to Library, select the PDF, then sync. See iTunes Help for more info about syncing.

Read PDFs - iPhone

http://help.apple.com/iphone/9/#/iphab2193d5

You can also sync your collections across devices:

Use iBooks with PDF documents on your iPhone, iPad, and iPod touch - Apple Support

https://support.apple.com/en-us/HT202075

• See collections, highlights, notes, and bookmarks:

   
   
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    On your Mac: Choose iBooks > Preferences, click General, then select “Sync bookmarks, highlights, and collections across devices.”

     
   
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    On your iOS devices: Turn on Sync Bookmarks and Notes and Sync Collections in iBooks settings.

     

Note: Any books or PDFs you import won’t appear automatically on all your devices. Only books purchased in the iBooks Store can appear everywhere via iCloud. To see imported books or PDFs on a device, sync the device using iTunes.

See your books on all your devices - iBooks Help

https://help.apple.com/ibooks/mac/10.10/index.html?localePath=en.lproj#/ibks86da b303

You could also use AirDrop, if your computer supports it, or email the PDF files to yourself and retrieve the email on your iPad.

Hope this helps. Thanks for using the Apple Support Communities!