Files such as PDF- and Word-Documents, Excel-Sheets, Powerpoint Presentations contain formatting information beside the readable text. This requires them to be transferred as binary files using CODE_FLAG=BINARY. This assures that XCOM does not check for End-of-line record separators and possible conversion from LF to CRLF or vice-versa which would corrupt the file.
The same is valid for any pictures, videos and Sound files which are to be transferred using XCOM.