I hace an original Acrobat Pro DC license. I cleaned my hard drive and now I don't find the way to use my licence again. I downloaded the software from the website, I Installed it, but How to write the serial number?
Acrobat DC doesn't have a serial number. What you have is probably an older version, like Acrobat 2015 or 2017. They used to be called Acrobat DC (hence the confusion) but that's now only used for the subscription-based version of the application.
So you need to find out exactly which version you bought, get the correct installer for it, and then install that and enter your serial number for it.
If you ever registered it with Adobe it should appear under your account.