Cross-Platform PDF Editor App with digital signing

I'm looking for a skilled mobile app developer to create a PDF viewing / editing app that has the capability of signing a PDF document with the mobile phones touch screen as well as digitally signing with a certificate (for instance .:PFX). Apps are required for both iOS and Android.

Key requirements include:
-Mobile app for both Android and iOS
- Ability to sign PDF with a handwritten signature using the mobile touchpad
-Ability to sign PDF using a digital PKI certificate (.PFX).
- User-friendly interface

Ideal skills and experience:
- Proven experience in developing mobile apps for both iOS and Android
- Strong background in handling PDF files
- Expertise in UI/UX design
- Familiarity with relevant programming languages and frameworks

Please provide a portfolio with similar projects. Thank you!