Japan Acrobat Pro DC 2015

in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.

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assuming you're using a valid adobe serial number (6 groups of 4 numbers), which is not clear because you purchased from a problematic vendor or adobe products, you have a mismatch between your installation file and serial number. ie, you can have a valid serial number, but if it's for program A and your installation file is for program B, you will see an invalid serial number message.

common mismatches are language/region (eg, serial number is for western europe and installation file is eastern europe), platform (eg, serial number is for a mac and installation file is win), license type (eg, education vs enterprise vs individual). and one of the most common is using a program installation file with a suite's serial number. eg, using a cs6 suite serial number with a photoshop cs6 installation file. that won't work even though photoshop is included in that suite.

you must use the (correct) suite's installation file (and you can choose to only install one program from the suite). here's official info about that error, http://helpx.adobe.com/creative-suite/kb/error-serial-number-valid-product.html

Anyone who buys software on eBay is asking for trouble.  Buyer beware. 

It's probably too late to get a refund from the seller.

Next time, buy directly from the software maker, not some unknown 3rd party on an auction site. 

https://www.adobe.com/acrobat.html