It is not supported to save docs to external SD card since the version 6.4.0.

In the lower CS version, to save docs to the external sdcard, please go to CamScanner Me-> “Doc Management” -> “Save to”. New scans will be saved to sdcard1/Android/data/com.intsig.camscanner/files.

If your phone has not been rooted, then you cannot change the default file path. Also, in order to avoid the sharing failure or unexpected data loss, it is suggested to save the docs to internal phone memory.