Hi Adam,
As per the description above, you are not able to combine two PDF files as getting an error 400, Is that correct?
Please try to combine the PDF files with the URL https://cloud.acrobat.com/combinepdf on a different browser like Chrome, or Firefox, or Safari, and see if this brings any difference.
If you are using the free desktop version of Adobe Reader, then sign out and sign back in with your current Adobe ID and password in Adobe Reader, and check for any pending updates of Adobe Reader from Help>Check for updates. https://cloud.acrobat.com/combinepdfRestart the machine after the installation of updates(if any).
You may also refer to the following KB doc to check the File types you can convert/combine How to Use Adobe PDF Pack
You may also refer to the following KB doc File conversion error when combining files
Conversion Failure while combining files
PDF Pack combine files doesn't work, conversion failure
Let us know how it goes.
Regards,
Anand Sri.