I already posted on what is docx file and how you can open it using the online docx converter i.e, to open docx file format, you have to convert docx to doc or docx to text file online. As an alternate solution for people who thought that online docx conversion was time consuming can check out this method.
Once again, Google docs comes in as a handy tool. Last year, there was a news that Google docs would support opening of Docx file format and currently, Google docs now supports the Microsoft docx format which is an added advantage as it is completely free and reliable.
Head over to the Google docs, make sure you are signed into Google account and now use the upload feature to upload the docx file you received from email attachment, your client or anyone. After finishing the process, you can open docx from the Google docs interface. Simple as that! No need to download any addon or convert docx to other format now.