Click the "+ Create" in the top right to add Supporting Documents to your Form
Supporting Documents Section in the left-hand sidebar (LHSB)
There is a specific section at the bottom of the form built to store the supporting documents. You will click and drag the supporting document component from the left-hand sidebar into the supporting document section. Similar to Fields there will be Settings, Conditional Logic, and the ability to Require a supporting document
Bulk Import for Supporting Documents
The Bulk Import option for Supporting Documents at the bottom right of the Supporting Document section