Global Form Script Issues

You won't be able to do this with anything out of the box in Marketo that I know about.  This requires a custom setup/process.

The use case of the need to direct a person to a unique asset on form submission means a global form may not be the best for you, completely IMO.

However, to answer your question, I would probably create a custom script to fire off upon form submission and evaluate against a UTM parameter (clearly defining the UTM and mapping it to what asset should be served up).

Well, your task is clear: the page that hosts the form needs to know the URL of the related PDF!

Whether that URL is supplied as a query param (as Jim suggests) or perhaps is supplied via a page variable set up in your CMS (for example, a WordPress site can have a page type that requires the page author to supply a string variable) is up to you.  The end goal is the same.

For a UTM param mapped to the field lastAssetURL in Form Editor:

MktoForms2.whenReady(function(form){
  form.onSuccess(function(vals,thankYouURL){
    var currentValues = form.getValues();
    document.location.href = currentValues.lastAssetURL;
    return false;
  });
});

For a page variable set using some other method:

var currentAssetURL = Your_WP_Variables.assetURL;
MktoForms2.whenReady(function(form){
  form.onSuccess(function(vals,thankYouURL){
    document.location.href = currentAssetURL;
    return false;
  });
});

Awesome stuff, we were able to use what you provided and get over to a global form. Thank you!