Is there any way to restrict access to a linked PDF or other document type? I understand that attachments aren't possible and landing pages are essentially the only way to go, but is there any roundabout way of keeping anyone who happens upon the URL from plugging it into the address bar and viewing the file? Also, is there a way to keep an in-browser linked PDF from being crawled by search engines?
If you mean restricting access to a PDF, ZIP, etc. uploaded into Marketo Design Studio then I believe the answer is, "no", that cannot currently be done. However, if you have access to a server of your own, you can implement some sort of referrer rule restriction on these types of files using mod_rewrite for Apache or URL Rewrite for IIS.
For instance, we generally upload our content to Design Studio, but if we want our partners to download proprietary or sensitive info, I upload it to a server that has a referrer rule stating that only a direct link from a page containing "evolveip.net" can have access to the file. This means that any page coming from www.evolveip.net, support.evolveip.net, or pages.evolveip.net can access the file, preventing direct linking of any kind.
If someone knows the direct address of the file AND knows how to inject a referrer into the meta info, they can technically circumvent this method, but its a heck of a lot more secure than Design Studio for serving up sensitive file downloads.