Good questions. First, anything which you want to rank in Google can be found in Google. Take for instance, I sometimes do searches for ebooks of certain topics I want to learn about for example, "how to write an ebook". I'd search "how to write an ebook filetype:pdf" and then I could potentially find direct PDF downloads from URLs and bypass any one looking to collect my email, name, etc. I don't check them all, but some may not have the PDF protected or blocked with a robot.txt file discouraging Google from indexing the page, but some do, yet their PDF can be found through a link, especially in WP sites. So, you raise a question about having your content indexed to aid your SEO efforts, right? If you're going to protect the PDF and want the content indexed then sneaky searchers like myself could find your content, otherwise, put the content on the page and the PDF and let Google index. I'd say majority of the people coming to your page would enter in their info to get the ebook(s).
I would, for the sake of saving your users more clicks and thinking, place the downloadable PDF on the Thank You page. Collect their info and eliminate any further confusion or clicks. "Thank you for taking interest in our ABC ebook. Please click below to download your copy now."
Hope this was a good answer! Cheers - Patrick