You canât. Once another person has wordfile.doc on their computer, a simple command prompt copy command makes another copy. Or with Windows Explorer select file and select copy. Boom! Another copy. It can be uploaded to GoogleDrive, OneDrive. Dropbox and other online file services. It can be transferred to a mobile phone. It can be uploaded to a web host via FTP. It can be saved to any drive media. internal hard drive, external hard drive, writeable CD, optical drive. It can be converted to another format online or with a desktop application. With Word 10 you can print a doc file to a PDF file. PDF file could be converted back to Word with more or less document intact depending upon quality of conversion.
So the best protection against plagiarism that I know of is that it's extremely rare (if not zero).