To combine two whole PDF records with each other, you should simply choose the PDF that you need to add to the ongoing PDF:
Click the page you want to add your other PDF after on the left. To merge the entire PDF, just select every page.
Open the Edit menu at the top and click Insert > Page from File.
Choose the other PDF you want to merge with the current one.
Your PDFs should now be merged. Click on Export as PDF from the File menu to save your new merged PDF.