If you're looking to try out Adobe Photoshop CS5 Extended, I recommend visiting Adobe's official website to download the free trial version or purchase a legitimate license.
If you previously purchased a license and need to recover your serial number, there are several official ways to find it:
Because CS5 is a legacy product (released in 2010), you may encounter modern activation hurdles: Server Connectivity
While using a trial serial number may seem like a convenient way to try out the software, there are risks involved: