All versions of Adobe InDesign CC up to 2018 are compatible with SignAgent Pro. If you do not own a compatible version of InDesign you will need to purchase Adobe InDesign CC.
UPDATE: InDesign has released the newest version, CC 2019, and current versions of SignAgent Scripts are incompatible with this new release. We're working on a patch that will allow compatibility and we'll let you know when that is ready.
Compatible versions of Adobe InDesign
* Adobe InDesign CC (up to 2018)
InDesign 2018 is available for an easy and free downgrade for anyone who has an active Creative Cloud subscription. You can see details on that here: Install Previous Version of CC Application
Incompatible versions of Adobe InDesign
Adobe InDesign CS6 & Earlier
All other version of Adobe InDesign are incompatible with SignAgent Pro™. You may be able to open the file. However, there is no guarantee that the scripts will function correctly.
You can learn more about InDesign here: http://www.adobe.com/products/indesign.html