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As far as I understand from your question, you want to upload a pdf file to preview it on your form. You can easily achieve this by using the widget called "PDF Embedder" Widget.
To do that,
1) Open the "Widgets" tab and search for PDF Embedder.
2) On the following widget settings window, define the PDF URL and check "Attach to Submissions" check box, if you want to add this file to your "Notification" or "Autoresponder" email.
If you refer something else in your thread, please let us know.