How to open docx files ?
.Docx files format have been introduced in the new Office & Word application to replace .doc file, however there are people who’s still using old Windows, therefore docx files are not supported, thus users may find it annoying for not being able to open it. Below are some tips (Depend on your operating system) on how to open a docx file :
Microsoft Office/ Microsoft Word (Older Version) :
Install the Microsoft Compatibility Pack in which it has the ability to support new file formats
Open Office
Open XML Translator in which it has the ability to read & edit docx files
What if I do not want to install the solutions mentioned above?
Try & search for DocX converter online, there are plenty of services ready for your service 
Another way is to ask the person to save as doc file before passing the file to you.