In Acrobat X standard or above, go File – Create – Combine Files into Single PDF. You can then add the files that you want to combine. In the combine files dialogue box, be sure to make sure that 'single PDF' is selected.
Alternatively, you could open the PDFs side by side, open the page thumbnails (on the left hand side) and drag and drop the thumbnails into the right place in the pages panel.
If you have a PDF open to start with you could try these methods
Open the pages panel on the left, select the dropdown menu and choose Insert pages – from file
Or select the PDF on your computer operating system (such as Finder[mac]), and drag and drop it into the PDF that is open in Acrobat
and lastly, Tools – pages – Insert from file
Hope this helps?