Is there an iOS app that takes a picture from a sketch on white paper and converts it to a drawing/line-art, kinda like that signature feature when editing PDFs? Googling for sketching alike apps result in Photoshop-ish apps, which is - if possible - not directly what I'm looking for.
If you have an Adobe CC account, you can use the Adobe Capture CC app to take a photo and vectorize it. The app is available for iOS and Android devices.
With the app, you can take a photo of a hand drawn sketch on your mobile device, then vectorise it, and send it to Adobe Illustrator (the desktop app) direct from your mobile device.
Of course you don't need any Adobe software to do something similar - it would be easy enough to take a photo, and open it in free software on your computer, such as Inskcape, and auto-trace the image to turn it into vectors.