I didn’t find this info by searching the forum until after locating the directory on my Windows 10 machine, so I thought I’d put it here for an FYI post. Once I knew the path I did find this post.
The system.DocumentsDirectory or Simulator Sandbox\Documents directory maps to:
C:\Users\<yourUserName>\AppData\Roaming\<yourAppName>\Documents
on Windows 10. This could be different for other Win versions.
Just thought i’d pitch in for windows 8.1.
On windows 8.1 its
C:\Users\UserName\AppData\Local\Corona Labs\Corona Simulator\Sandbox\APP\_NAME
In computer search bar just type in “%appdata%” and then you’ll see 3 files. Follow steps above to find the app documents directory.
–SonicX278
We document where Corona’s sandboxed directories are located for Win32 desktop apps in the following guide. There may be other info there that may interest you too. So, check it out. 
https://docs.coronalabs.com/daily/guide/distribution/win32Build/index.html#building-running
And for your quick reference…
system.DocumentsDirectory: .\AppData\Roaming\<CompanyName>\<AppName>\Documents
system.CachesDirectory: .\AppData\Local\<CompanyName>\<AppName>\CachedFiles
system.TemporaryDirectory: .\AppData\Local\<CompanyName>\<AppName>\TemporaryFiles
The <AppName> and <CompanyName> are what you enter into the build window. Company name is optional, but it’s recommended that you set it. App name is required.
Note that Windows will automatically synchronize files under the “AppData\Roaming” folder with other Windows machines the user logs into if it’s part of a Windows domain and using Active Directory. This Windows feature is typically only used by large enterprises and universities. It’s not something home users would use. The files under the “AppData\Local” directory will always remain local to the machine (not synchronized).
The directory @SonicX278 mentioned only applies to projects ran under the Corona Simulator. Not to Corona built Win32 desktop apps.
I hope this helps!
Ahh then I must have misunderstood.
–SonicX278
Thanks for the other paths!
Just thought i’d pitch in for windows 8.1.
On windows 8.1 its
C:\Users\UserName\AppData\Local\Corona Labs\Corona Simulator\Sandbox\APP\_NAME
In computer search bar just type in “%appdata%” and then you’ll see 3 files. Follow steps above to find the app documents directory.
–SonicX278
We document where Corona’s sandboxed directories are located for Win32 desktop apps in the following guide. There may be other info there that may interest you too. So, check it out.
https://docs.coronalabs.com/daily/guide/distribution/win32Build/index.html#building-running
And for your quick reference…
system.DocumentsDirectory: .\AppData\Roaming\<CompanyName>\<AppName>\Documents
system.CachesDirectory: .\AppData\Local\<CompanyName>\<AppName>\CachedFiles
system.TemporaryDirectory: .\AppData\Local\<CompanyName>\<AppName>\TemporaryFiles
The <AppName> and <CompanyName> are what you enter into the build window. Company name is optional, but it’s recommended that you set it. App name is required.
Note that Windows will automatically synchronize files under the “AppData\Roaming” folder with other Windows machines the user logs into if it’s part of a Windows domain and using Active Directory. This Windows feature is typically only used by large enterprises and universities. It’s not something home users would use. The files under the “AppData\Local” directory will always remain local to the machine (not synchronized).
The directory @SonicX278 mentioned only applies to projects ran under the Corona Simulator. Not to Corona built Win32 desktop apps.
I hope this helps!
Ahh then I must have misunderstood.
–SonicX278
Thanks for the other paths!