If you are subscribed to PDF Pack service then you can do so by Visiting this Link - https://cloud.acrobat.com/combinepdf
Here's a helpful Link - https://helpx.adobe.com/acrobat-com/kb/using-pdfpack.html#main_Combine_files_to_PDF
Alternatively, you can also use Acrobat software for combining files ( Download a FREE30 Day Trial using this link:- Download Acrobat products | Standard, Pro | XI, X )
IF you use Acrobat software then you would have to follow this navigation path -
'File' -> 'Create' -> 'Combine files into single pdf' -> 'Add Files' button on Top left of the Combine Files dialog Box -> and then choose the files to be combined and click 'Combine Files.
Hope that helps with your query.