If you still have a valid 2015 serial number, you can typically find the installer through your under "Order History" or "Downloads." To help you get the right version for your needs:
offer public download links for this obsolete version. Users who previously purchased it digitally should check their official Adobe Account Adobe Acrobat Pro Dc 2015 Classic Download
A desktop-only version purchased with a one-time payment (perpetual license). It received regular security updates and bug fixes but did not receive new functional features after its initial release. Key Features of the 2015 Release If you still have a valid 2015 serial