How to reinstall Acrobat creative suite in a new computer-it says serial number is in use

It's 2020 and this is where things stand right now. CS1, CS2 and CS3 are dead & gone. The activation servers were removed from service years ago. Even if you find an installer, it's impossible to activate the software.

CS4, CS5 and CS6 are on life support. But they can still be activated providing you have the following:
1. original media (disks or downloads) you received at purchase and a way to read them,
2. a valid serial number for that version, language and platform (Win/Mac),
3. an older operating system that can support legacy 32-bit apps.
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NOTE 1 : Creative Suite installers won't work on macOS Sierra, High Sierra or Mojave without terminal workarounds. See the help article below.
https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html

NOTE 2: Creative Suite cannot run on Catalina. Either downgrade your OS, set-up a virtual machine or get all new, 64-bit software. Your choice. https://www.adobe.com/creativecloud/plans.html

NOTE 3: Legacy Creative Suite is discontinued & unsupported. Use at your own risk. Customer support cannot help you with anything other than an activation count reset. 

For security reasons, you really should use a current version of Acrobat, not a legacy version.

FREE Acrobat Reader -- view, comment, print & sign PDF.
https://acrobat.adobe.com/us/en/acrobat/pdf-reader.html

Paid Acrobat Pro DC -- the complete solution for creating, editing, signing, exporting and saving PDF.
https://acrobat.adobe.com/us/en/acrobat/acrobat-pro.html

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Beware of fake Adobe reps who may contact you privately offering to sell you software or take control of your computer. It's a scam, run away!
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3 Easy Ways to Identify Genuine Adobe Staff
https://tinyurl.com/10791730