New tab bar icons in PDF format... Guidelines to create custom?

Question

I have noticed that in recent versions of Xcode, the project template for "Tabbed Application" creates a project where the tab bar icon images (dubbed "first" and "second", for the default two tabs/view controllers available) are actually stored in .pdf format, not .png, and displayed in the Assets.xcassets folder with a resolution of "All" for use as "Universal" (instead of -say- a resolution "@3x" for use as "iPhone app iOS 7-8", like the app icons).

I assume this is to provide a vector graphics-based resource that is truly resolution-independent and hence supports all sorts of devices.

My question is: What are the guidelines to create custom resources in this format? I guess it should start by creating a custom outline in Adobe Illustrator and exporting it in pdf format, but, What size and units should I use? Are there any other things to look out for? I haven't found anything in official Apple documentation and a web search for "ios tab bar icon pdf" only gives links to websites providing clip art etc.

Note: I have thought whether to post this question here or in Graphic Design Stack Exchange site, but it still has to do with the Xcode IDE and developing iOS apps, so I place my bet here. Feel free to vote for closing it if you think otherwise.

Answer 1

I understand MirekE's answer as I lived the challenge and now have a solution. But for anyone else, here's a real world example.

I create all my line icons (or anything of your choosing) using sketch (highly recommended) using a 30 X 30 Artboard then applying the drawings inside. I make the background transparent and then set my export to 1x size and apply the PDF format. Visible in this screenshot.

Then it's a simple import into Xcode > create the object in Images.xcassets > select the image group > choose single vector from the pulldown scale factors (in the attributes inspector) > then just drag and drop the imported image in the single place holder.

I've applied this approach to all my custom tabbar items and the result has been stellar.