By default, PDF2XL converts each document into its own individual Excel sheet, but if you want to append a file to an already existing spreadsheet, you can do so using the "Add to existing sheet(s)" option in the Conversion settings.
Go to the Convert tab and click on the arrow under the Excel option; select "Excel File", then convert your first PDF.
Open your next PDF, then go to Settings (in the Convert tab) and in the "Conversion to a file" section, check the "Ask for a filename" checkbox.
Underneath that, in the "When file exists" dropdown, select "Add to existing sheet(s)".
Click "OK", then the "Convert" button (you will need to apply your layout first).
You should see the file explorer window pop up - you will need to select the file that you want to append the current conversion to (the one you converted in step 1).
Once the conversion is complete, you should see your new conversion at the bottom of the original Excel file.