DocX is a compressed zip container file. Make a copy of the original docx file. If you unzip it, there is a subfolder named word with a file named settings.xml. You can open settings.xml with a plain text editor.Within that there is a block of text:

<w:documentProtection
w:edit=""
w:enforcement="1"
 w:cryptProviderType=""
 w:cryptAlgorithmClass=""
 w:cryptAlgorithmType=""
 w:cryptAlgorithmSid=""
 w:cryptSpinCount=""
 w:hash=""
 w:salt=""/>

You can either change w:enforcement value to 0 or remove the block

Source: How can I unlock a Microsoft .docx document? – Super User