We are using the latest Landing Pages on WP 4.9.5 and we are experiencing an issue when people click the download button on the Inbound Form on the landing pages. Instead of redirecting them to a PDF a blank white page with a -1 appears. The URL for the PDF that is configured in the Redirect URL field in the Inbound Form works by itself. Also, for some reason everything works if we are logged into the WordPress site. This only happens when we are not logged in.
I tested for conflicts in both the theme and the plugins as you suggested. I looks like there was an issue with the WP Super Cache plugin. I disabled that and everything started working again. So in production I cleared the cache, modified the settings of the plugin to not cache the .pdf that were supposed to load. Now to see if that makes lasting differences.