Handing a PDF to another app with Open in, in PDF by Zaweba
File > Open in hands the PDF you have open to another installed Zaweba app, and hides itself when there is none. When and why to use it.

No single program does everything, and PDF by Zaweba doesn't pretend to. When a document needs a job done that belongs in another Zaweba app, File > Open in hands the open PDF across, so you don't have to find the file again in another program's open window.
Handing a document across
Open File > Open in and you see the other Zaweba apps installed on your PC that can take the document. Choose one and the PDF you are looking at opens in it. It is a direct hand-over, the same file, without saving a copy somewhere and browsing for it.
If there are no other suitable Zaweba apps installed, the item hides itself. There is no point offering an empty menu, so if you don't see Open in, that is the reason. Install another Zaweba app that works with PDFs and it appears.
Why it helps
Hunting for a file is one of those small jobs that eats time. You have the document in front of you, but to open it elsewhere you have to remember which folder it came from, go there in the other program's Open window and pick it again. If it came as an email attachment, it may be in a temporary folder you'd never find on purpose. Open in skips all of that.
It also keeps you thinking about the job rather than the files. You read the PDF here, decide it needs something another app does better, and move on in one step.
Save first
If you have made changes in PDF by Zaweba, save before using Open in. The other app works with the file as it is saved on disk, so unsaved changes won't be there. A quick save first means the other app gets the version you have been working on.
Coming back is easy too. PDF by Zaweba doesn't lock its files, and when another program changes the open file, the app can reload it. So you can hand a document over, work on it in the other app, save there, and see the new version back in its tab here.
Part of a family
Open in comes from the shared core that Zaweba apps are built on, the same core that provides the Help menu with feedback, updates and the list of other Zaweba apps. That shared base is why the apps feel similar and why they know about each other.
Help > Other Zaweba apps shows what else is available, which is a good place to look if you find yourself wishing PDF by Zaweba did something it doesn't. Often another app in the family covers it, and Open in is the bridge between them.
Using it well
Think of Open in as the last step of one job and the first step of the next. Check the document here, where it opens quickly and searches fast, then hand it on for the specialist work. Bring it back afterwards to check the result.
It is a small menu item, and on a PC with only PDF by Zaweba installed you won't see it at all. But once you use a few Zaweba apps together, it quietly turns them from separate programs into a set of tools that pass work between themselves.