Your old version of Acrobat is no longer supported. You may be able to install it but the activation servers for that platform were retired.
If for any reason you can't afford at this time a monthly subscription for the new Adobe Acrobat Pro DC version then you may have to contact Adobe customer care directly and see if they can or will provide you with some limited offline support for your product.
Be advised that you still need to be online for a registration of such product.
Customer Care agents cannot help with this. Adobe removed the activation-free installers and replacement serial numbers. Acrobat 8 is officially dead now.
Acrobat Reader DC is free.
If you need more than that, you can buy a 1 year subscription to Acrobat Standard (Windows only) at USD $12.99/month or Acrobat Pro DC (Win or Mac) for USD $14.99/month.