how to reduce space between words in word docx

You didn't specify your operating system when you posted. My reply assumes you are using Word for Windows.

The width of a space character is determined by the font you are using, not by the program.

If your text is left- or right- aligned, you cannot change just the size of the space character. You can use Home>Font>Advanced>Spacing>Condensed on selected text or in a typestyle, but all characters will be affected, not just spaces.

If your text is justified, Microsoft's crappy default algorithm adjusts only the width of the space characters. Consider using left-aligned text instead, as this is known to be more readable as well as looking better. It's possible, using a VBA macro, to put a document into Word 2010 compatibility mode, then apply the WordPerfect justified text algorithm, which looks a lot better. However, by default, Word will try to upsave compatibility-mode files and when it does, the justified text will go back to looking bad.

Unicode fonts contain a Thin Space character that you could use as an alternative to the regular space. Here's a page with more space character alternatives: Unicode spaces