How to Merge PDF Files With Adobe Acrobat DC

Step 1: Open Acrobat DC and select Tools > Combine Files.

Step 2: Select Add Files to add as many files as you would like. Select Add Open Files to include documents you’re currently viewing.

Step 3: Adjust the order, including that of individual pages, by dragging and dropping each to the desired location.
Then select Combine to complete the process.

Credit : https://www.lifewire.com/combine-pdf-files-4155148