You can select up to 100 documents to download at a time.
If you register for your personal account you can change the general settings so that your search will display up to 50 results per page.
Select the box at the top of the search results to select all of the documents on that page.
After you check the box the folder next to it will show that you have selected 50 documents.
Scroll to the bottom of the results to move to page 2 of the results and repeat the steps to add the next 50 documents to your folder. Your download is limited to 100 documents at a time. Select the three dots next to the download icon to make any changes to your download preferences.