Custom Document Signing App Development

I'm looking to develop a custom application for both Android and iOS platforms with the primary purpose of facilitating document signing. The app should allow users to mark specific areas on PDF documents where signatures are required and enable sharing with other users for signing. I prefer to avoid using costly APIs, so the app should be developed from scratch.

Key Requirements:
- Support for PDF document signing
- Users can mark signature areas on documents
- Document sharing functionality for obtaining signatures from others
- Notifications to alert users when documents are signed
- User authentication via email and password

Ideal Skills and Experience:
- Experience in mobile app development for Android and iOS
- Proficiency in handling PDF documents within mobile applications
- Strong understanding of user authentication systems
- Ability to implement notification systems within apps
- Experience in developing apps without relying on third-party APIs

I'm excited to see innovative solutions that meet these requirements and provide a seamless user experience.