You can attach a PDF to a custom form but not an Excel. For your situation, I recommend adding a Descriptive Text custom form field and you can have verbiage for instructions on what to do and link to the excel. Then the users can submit a request but before they do, click the excel link to also fill in the excel that you need. Seems like only option to me. Here is screenshot of when you add a descriptive text field, you have your text and link. Note the whole text will link, not just a portion of the text but that shouldn't matter really.
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