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If you have a file with extension .docx that means you hav a word file created in word 2007. Unlike word file created by 2003, 2000 it will create a file with .doc extension but if you have created .docx with word 2007 onwards you can open the file with office 2003 you need to have Office 2007 or you can download Word Viewer  or Word Reader to open .docx file

You can donwload the  Word Viewer from Here .

Once you download Word Viewer you can able to open .docx file

Compatibility Pack for the Word, Excel, and PowerPoint 2007 File Formats

To reduce file size, improve reliability, and enhance integration with external sources, Microsoft has added new file formats to Word 2007, Excel 2007, and PowerPoint 2007. To help ensure that you can open files in these new formats, Microsoft has developed a Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. By installing the Compatibility Pack after you install the Word Viewer, you can view documents that are saved in the following new document formats:

• Microsoft Office Word 2007 document (.docx)
• Microsoft Office Word 2007 macro-enabled document (.docm)

Note: When you use the Word Viewer to view files that are saved in these formats, you will see a Word version of the file. The following Word 2007 items will be affected:

• Features that do not exist in Word 2003
• Layout that uses properties that do not exist in Word 2003

For more information on specific changes, see Open a Word 2007 document in an earlier version of Word. Compatibility Pack for the Word, Excel, and PowerPoint 2007 File Formats

To reduce file size, improve reliability, and enhance integration with external sources, Microsoft has added new file formats to Word 2007, Excel 2007, and PowerPoint 2007. To help ensure that you can open files in these new formats, Microsoft has developed a Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. By installing the Compatibility Pack after you install the Word Viewer, you can view documents that are saved in the following new document formats:

• Microsoft Office Word 2007 document (.docx)
• Microsoft Office Word 2007 macro-enabled document (.docm)

Note: When you use the Word Viewer to view files that are saved in these formats, you will see a Word version of the file. The following Word 2007 items will be affected:

• Features that do not exist in Word 2003
• Layout that uses properties that do not exist in Word 2003

For more information on specific changes, see Open a Word 2007 document in an earlier version of Word.