To create your field map, select the PDF Form in the main window of the add-on, then go to Settings>Field Map....

To map a field, double-click a row of the grid or right-click it and select the Edit... command.

You may map to a contact, company, group, opportunity or product field.
Mapping to an opportunity or product field
If you map to an opportunity or product field, you will be prompted to select a row number for the field, in case you want to merge more than one item. When you run the merge, you'll be prompted to select one or multiple items and the row number determines which item will be merged into the field. For instance, if you select 2 products, the field below will be filled with the Name of the first selected product.
