How do I activate Export PDF on 3rd computer?

Hi Noram23125202,

As mentioned above, that you have Export PDF subscription so I have checked your account accordingly and found an active subscription of PDF Pack not the Export PDF.

PDF Pack also allows you to "Export" the pdf files. As its an online subscription so you can use it from any computer you want to.

https://cloud.acrobat.com/exportpdf

Also, you can install the desktop application Acrobat Reader DC and continue using it.

Adobe Acrobat Reader DC Install for all versions

You just need to "Sign In" with your Adobe ID under which you have the service registered.

Hope this helps.

Regards,

Akanchha