β Iβd like to know if GeneratePress would allow for a hook to open /brochure.pdf automatically when {onLoad} /thank-you page.
Hooks are simply for inserting HTML tags.
What you need is a script or plugin that actually pops up the HTML element that presents the PDF and another script or plugin to actually let the site embed the PDF to be made viewable within your page.
For the PDF viewer/embedder, you can use this – https://wordpress.org/plugins/pdf-embedder/
For the modal/pop-up functionality, you’ll need a script like the codepen I’ve linked on my previous site and/or find a plugin that does that.