The Check Spelling in Files option available from the Project contextual menu enables you to check spelling on multiple documents:
You can choose the following scopes:
Spell check in all opened files.
Directory of the current edited file.
Scope of the current edited DITA Map.
All files from the current project.
Selected files from the current project.
Specify a custom path.
You can also choose a file filter, decide whether to recurse subdirectories or process hidden files.
The spell checker processor uses the options available in the Spell Check Preferences panel.