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If I understand you correctly, you wish to add a link into your form from which your users will be able to download additional files.
You can add any links to your forms in the Text field. When you add text box field to your form double-clicking on it, or clicking on the Edit HTML button of that field.
When field is opened for editing add text for your download link select that text and click on Insert/Edit link option and paste URL of your link, click OK and Complete.
Also file that you want your users to download need to be hosted somewhere online.
Hope this will help. Let us know if you need further assistance.