Hope you are doing well and sorry for the trouble. As described, you are unable to open the PDF and getting the error 'Adobe Acrobat could not open the file because either it is not a supported file or..'
Would you mind sharing the the workflow/steps you did to create the PDF file from Word and to combine the PDFs together?
Please try to create the PDF from the Acrobat Ribbon present in MS Word or from the File menu in Adobe Acrobat DC. File > Create > PDF from file.