We are moving to a new website with advanced search functionality. That means our assets will come up in search; but will be pulling up the PDFs etc. that i would rather are behind a landing page. I like the idea of them being indexed and searchable, however, how can I keep them behind the gate? Any suggestions?
If you have the assets indexed (like a PDF), the search engine will reference that PDF in the URL and will directly link to that asset (defeating the purpose of the date). So you will need to decide if you really want them indexed. As Josh menttioned, you can certainly have the landing pages indexed (be sure to include the keywords in the page).
You can largely control the behavior of a search engine by use of a robots.txt file (http://www.robotstxt.org/robotstxt.html or https://support.google.com/webmasters/answer/6062608?hl=en).
Otherwise, I think you are stuck and need to make a choice between the search engine or gating the asset.