As you are thinking of to upgrade the service to use for annotation and editing, would like to infor you that annotation service is available for free in Adobe Acrobat Reader DC. Adobe Acrobat Reader DC software is the free, trusted global standard for viewing, printing, signing, sharing, and annotating PDFs. You may use the application to fill the PDF forms.
The application is also available for mobile devices. You may install Adobe Acrobat Reader from Google Play Store on Android and App Store in iOS devices.
To edit a PDF you will require Acrobat Pro DC susbcription. The susbcription will work on all devices: Acrobat Pro DC for desktop and Adobe Acrobat Reader for mobile. However, on mobile application there is a limitation of access to the tools feature. You may edit PDF in mobile application, however, you will not find all the desktop edit feature on mobile device.
For more information about applications, please refer to the information povidd below: