When I drag a pdf file to bike, I got a nice link and url is file:///path/book.pdf . I can open this link in browser or other tools, but I can’t open it in bike? The open link function didn’t work.
I use the “free” version, edit link function is blocked but I can open web url.
To create the link its easiest to just drag and drop the file into Bike’s outline.
To open a link you need to click the trailing “link” button that looks like this: 
Bike won’t be able to open the file directly because of macOS Sandbox restrictions, but what it will do is reveal the file in the macOS finder when you click the link. I hope that helps.