Linking and locking downloadable file behind marketo forms on landing page

"Locking" is a bit optimistic here, given that Marketo-hosted assets don't require a login and if you know the asset URL, you can provide it to others (of course, you can also provide the asset itself if it's just a PDF!).

But if you want to gate the asset without redirecting to it immediately, you'd still set the PDF's URL as the Thank You URL, but use the Forms2 API to stop the page from redirecting entirely to the PDF (which isn't the greatest UX anyway). Then set the href of an existing <a> tag to the PDF URL.

A compressed example:

<style type="text/css">
#assetDownload { display: none; }
#assetDownload.downloadReady { display: block; }
</style>
<a id="assetDownload" href="#">Download Now</a>
<script>
MktoForms2.whenReady(function(form){
  form.onSuccess(function(vals,tyURL){
    assetDownload.href = tyURL;
    assetDownload.className += ' downloadReady';
    return false;
  });
});
</script>

If you want a page>Form>ty page with link then

1. Create page
2. Create form
3. Add form to Page
4. Create TY page with URL to PDF
5. Approve pages
6. fill out form

you may need a Trigger Campaign to manage the journey based on this form fill.

Recommend putting PDF on your main website for load balancing.

Also, check docs.marketo.com and University as there are good examples of the above.