Is it effective to place eBooks and other PDF files (like samples etc.) on your website from the standpoint of SEO and marketing? How can one get the best benefits out of these tools?
Giving free eBooks or samples is a good way to get subscribers make your mailing list longer. I don't know how it affects SEO but I'm sure it is a good way of marketing.
Yes, everybody loves a freebie guide, if it is of particular interest for their niche (ie: fishing - download this PDF which has the top 5 fishing spots in America)... Your 2nd part to your question is the key part: How to maximize your benefit of doing this? First, you want to make sure you get their email address in exchange for the PDF. That should be a no brainer. Next, inside the PDF, you want to have your affiliate links. Make sure they are cloaked. If you do not know how to cloak your affiliate links, search google for 'php redirect' and learn. Finally, at the end of your pdf, you want to have a link which funnels them into a big $$, such as a OTO one time offer - "stop, you are only going to see this offer ONCE - if you do not act now, you will instantly regret it..." "your chance to get the top 5 fishing spots in America, including the top 5 all you can eat buffets, for you and that nasty wife of yours..." - you get the idea.
Getting subscribers is one of the best ways to make money, and you can do that by offering them free ebooks and things like that if they subscribe. Then you can make money later on with your list of subscribers.
Not only to get subscribers, giving away free ebooks and give you additional visitors to your site. In your ebook, write: anyone can share this ebook as long as the content is unchanged.
I've seen PDF raports etc. used most often as follows: "enter your email address, you'll get a mail with a download link". Basically, using a document as a bait.