Adobe Reader Touch does not support Save or Save As yet. But we are working on adding the support in a future release.
How did you get PDF files "in Adobe"?
If you already have the PDF files on your device, you can simply use Microsoft File Explorer on the Windows 8 desktop to copy them to your USB flash drive.
If you tap/click on the link to a PDF file in Microsoft Internet Explorer (Windows Store app, i.e. Metro version), IE will prompt you to "Open" or "Save". After you choose the Save option, you can specify your USB flash drive as destination.