How do I stop the Cloud from downloading all my documents to a second computer?

Are the documents you are asking about on iCloud Drive or on your Desktop and Documents folder?

To stop the syncing of iCloud Drive and Desktop & Documents, you do not need to sign out of your AppleIdD, but just turn off selectively any iCloud Services you do not want to sync to your second Mac on your second Mac. Turn off AppleID > iCloud Drive > Options: Desktop and Documents, then iCloud Drive.

Then you can use still iCloud to sync your Mail and Contacts, and similar, but not the documents.

Once the services have been disabled on your second Mac, you can delete the synced documents and data from your second Mac, that you do not want to keep on that mac.

When a Mac and other apple devices are signed in with same Apple ID and password on the same network , if you open system preferences > iCloud and uncheck the box of iCloud Drive a pop up window appears , showing to keep a copy of documents , and remove from the Mac .

( a ) if you had clicked on keep a copy , they will be archived in home folder that can be viewed by clicking on finder in the dock , take cursor on top menu bar > Go > Home > your user name > iCloud Drive ( Archive ) .

Apply the above method in primary iMac , go to secondary Mac , the documents will take time to sync or if they are completely synced with high network speed drag and drop the desired documents in some other folders see the last paragraph of the article https://support.apple.com/en-gb/guide/mac-help/mchl1a02d711/10.14/mac/10.14

• On your Mac, drag items from iCloud drive to another folder in the Finder sidebar or on the desktop, such as Downloads, Movies, Music and Pictures. 
• The items are copied to your Mac and removed from iCloud Drive on all your devices set up for iCloud. If you change your mind, you can drag items back to iCloud Drive.

The original documents ( as you are dragging and dropping few of documents and not all of them ) will also be removed from primary iMac , but archived in its home folder , you can safely delete the documents from finder > documents - from the secondary Mac .