Can .DOCX file be read by MS Word?

-Yes, .docx is a newer file format used by words 2007 and upwards.It depends on what version of Word you have. .docx is the default format for Microsoft Word 2007 and above. If you want your file to be readable by all version of Word since 2003 save it as a .doc file instead of a .docx.
.docx IS word's default format, so yes word is meant to use that format.