Hi, that's a lot of variables and answers may therefore vary from different people, but here's my best shot:
**1. If we don't offer the PDF option, people would have to visit our site to read the content (unless they bought a hard copy). **If the goal is getting more readership and attention for these books then I think you should provide a PDF.
**2. If visitors were able to download a free PDF, they wouldn't need to return to our site to read it. **No, but the benefits of (1) above apply I think.
**3. If our corporate clients (nearly all of our clients are corporations) could download a PDF, they could then post it on an intranet instead of posting a link to our site. **I think you could put information in the PDF and on the page where it can be downloaded from to say that you do not give permission for the PDF to be hosted on another site. You cannot guarantee that they wouldn't but I think it is unlikely a corporate would want to be seen to be going against your wishes on that. Alternatively, you could say that the PDF can be hosted on other sites, providing they include a Follow link to your site attributing your site as the source.
**4. In general, do you think a visitor would be less likely to link to our site if he or she were able to download the PDF? Or would the appeal of the PDF option make it more likely that people would visit and link to the site? ** Yes - the second option
5. Also, if we offer the PDF option, are there any SEO issues related to duplicate content? Not really, if posting on other sites they include a link to the source of the PDF on your site
**6. Finally, if we did offer the free PDF download, would you recommend that we ask for an email address before giving the PDF? **It depends what your motivation for doing so is. If you want to build an email list that you can then send newsletters and special offer emails to, then yes - as long as you make it clear that is what you will do and you get the visitor to click a checkbox confirming they accept that condition - although of course you have to give them the option to unsubscribe at a later date when you send newsletters and offer emails.
I hope that helps,
Peter