To conserve disk or SSD storage space and time, you can save files directly to Dropbox from your computer programs that want to save to your Documents folder. This requires simple commands in Terminal for Mac OS or a small settings change for Windows.
For Mac OS, open Terminal (in Utilities) and type cd Dropbox. Press enter, and then type ln -s ~/Documents /Documents. Hit enter again to complete the process.
To segregate the Document folder of multiple users, or home machine from work machine.
To be clear you can use any file under the Dropbox folder if you want to segregate your work machine from your home machine. Just add the folder you want to use as the target under Dropbox.
To save the Document folder from a two different Mac machines as above type cd Dropbox at the command line in Terminal on the home Mac. At the terminal type mkdir HomeDocuments. Then type ln -s ˜/Documents /HomeDocuments. Hit enter to complete the process. The form of this symbolic link command (ln -s) is source directory followed by target directory.
If you do not want this HomeDocuments folder, sub-folders, and included files showing up on your work Mac you will have to use Selective Sync on your other Mac and uncheck the HomeDocuments folder. This process can be repeated from a work Mac using a different folder name (WorkDocuments).
The use of symbolic link (ln -s) can be handy as the user can even choose subfolders in the Documents folder and send files to Dropbox while leaving other subfolders resident on the machine without linking. One word of warning is that moving the source breaks the link.
It is more cumbersome on Windows machines and the granularity is less elegant. In Windows, right-click on your My Documents folder, hit Properties and click Move. Then select your Dropbox folder.