To add a document to a cardfile, view the cardfile, then drag and drop the file (from the drive/network) directly onto it.
A document added in this way would normally be flagged as Hidden, meaning that it won't be displayed in a document list, unless the Show Hidden icon is selected.
With the cardfile in View or Edit mode, select the Add Document from Clipboard icon in the ribbon:
Once added, the document will be accessible via that cardfile’s Documents tab (in the footer). A number will be displayed on the tab to show the number of documents linked to the cardfile. That number will increase as more documents are added to the cardfile.