Using Word to merge PDFs is NOT a good idea. Word would need to convert the PDF which will impact the page layouts. Unless this is a VBA programming exercise I would be using PDF-specific tools for this task.
You should be able to find free or paid tools to allow you to merge PDFs. On the paid front, Adobe Acrobat or alternatives such as Nuance Power PDF will help you merge PDFs.
Free tools also exist and can be found with a google search. You can also use online tools such as
PDF Merge - Combine PDF Files - Free Tool to Merge PDF Online