There are lots of PDF scanning and editing apps in the App Store and the choice of what you use is completely up to you. I'm going to recommend the ones that I've found to work great in the past. I'm a huge fan of Readdle software and the workflow I use consists of two apps, Scanner Pro and PDF Expert. I use Scanner Pro for times I have a document I need to scan in and sign. If you already have the document in your email or somewhere else on your device, you won't need a scanning app like Scanner Pro, only a PDF editing app like PDF Expert. Same things goes for if you have a physical copy you can sign and just scan, just pick up Scanner Pro.
For scanning:
For editing, annotating, and signing:
Use this set of directions if you have a physical document you need to sign and then scan to send.
Use this set of directions to not only sign, but edit and annotate documents, as well as complete fillable PDF forms.
It's worth noting too that we've only went over signing documents with PDF Expert but it's capable of a lot more including filling out forms. The steps are the same, you're just going to use different tools inside the app to do so.
That's all there is to it, you're done! Next time a printer or scanner isn't convenient, you'll be prepared for anything! These types of apps are extremely valuable tools for anyone who deals with signatures and contracts regularly such as real estate agents, building contractors, and self employed individuals who regularly need customer signatures. Save yourself some time and the cost of printer ink and save it all on your iPhone or iPad!