For the OSes to share common folders, you have to edit the fstab file to mount the Windows partition on boot, then change your default documents folder location.
Mount Windows partition on boot
This steps are taken from this answer which also list terminal-only instructions. Here i will just copy the visual way of doing it, for easy access.
Since the answer was for ubuntu, you have to install the Disks application fist, cause elementaryOS does not come with it.
Open the terminal and run this command to install it:
sudo apt install gnome-disk-utility
Once opened, on its main window, "Disks" graphically shows you your current partition layout.
Now simply choose the NTFS partition of windows that you want to automatically mount on boot, then click on the small gears icon slightly below it. From the menu choose:
‘Edit Mount Options…’
From the next window that is shown, move the slider button to the left, next to the ‘Automatic Mount Options’ label, to gain access to the settings. Keep the check-marked option ‘Mount at startup’. Select/Fill the next four options if you wish, like "Mount Point". For example:
Note: $Luser is your elementaryOS user.
- Now reboot.
Changing the Documents folder
The locations of every user directory are in "~/.config/user-dirs.dirs".
Open this file in scratch:
Search for this line in the file:
Replace "$HOME/Documents" with the path to the Windows Documents folder. So it would be somehing like this:
Note: $Wuser is your windows user.
- Now just copy all your files in your old documents folder to the new one, then reboot.