A _DOC file is a Microsoft Word document with the .doc extension changed to ._doc. It is a .DOC file, which may include formatted text, images, graphs, tables, and charts. _DOC files may be renamed to "._doc" files by an email program to prevent the file from being blocked by an email service.
Another reason why a _DOC file may be created by an email program, such as Windows Live Mail, is if the filename ends with a "." before the "." in the extension. For example, the email program changes sample..doc to sample._doc. This also commonly occurs with .DOCX, .XLS, and .XLSX files.
You can rename _DOC files by changing the "._doc" extension to ".doc". After renaming the file, you can open it as a standard DOC file. Some programs that can open DOC files are Microsoft Word, Corel WordPerfect, LibreOffice Writer, and Apple Pages.