Wordpress calculator plugin customization

I am using a 3rd party WordPress plugin (https://codecanyon.net/item/ez-form-calculator-wordpress-plugin/7595334) to create a custom calculator. A preliminary demo of the current project can be found here: https://www.digitaleventcarboncalculator.com/

I need help customizing this plugin in two ways:

1. The plugin natively supports creating a PDF version of the final result to be emailed to the visitor. However, it does not support downloading of this PDF directly from the site. I will need customization so that the PDF URL can be stored within an HTML button to be displayed on the calculation results page.
2. On Step 3 of the calculator, when choosing 'Yes' to the question, a group of additional fields are shown, including two percentage fields. I need these fields to interact so that they both equal 100% and if the user adjusts one field, the other is updated simultaneously. This is not supported natively by the plugin.

The calculator supports Javascript and additional documentation can be found here: https://ez-form-calculator.ezplugins.de/documentation/

Customizations must be done in a way so as not to effect updates to the plugin, theme or Wordpress core in the future. Any plugin customizations must use a hook or method in which the plugin files themselves are not directly edited.

Any additional questions regarding the scope of the project are welcomed.