Just jumping in to say I'm having the exact same issue as others described here. Otherwise enjoying using Orion but this is frustrating. Particularly, as other commenters note, when downloading large PDFs, or from a slow server, or via a series of redirects. I've also noticed this issue when PDFs have been previously loaded on an inactive tab, and when I switch back to it, the page is blank. This is especially frustrating if you are, like I am right now, in and out of internet access.
I'm used to downloading PDFs and not viewing them in browser. If this is not yet resolved I would appreciate the option to treat PDF links as a download and not default to opening them in browser. (though I suppose I can just revert to 'download as' pdf links, but that isn't possible with indirect links)