If you don't set spacing in your input xhtml p, default css values will be used, and applied expressly on each w:p.
I think see com.openhtmltopdf.simple.extend.XhtmlNamespaceHandler and 
https://github.com/plutext/docx4j-Impor ... andler.cssSo try changing that.
A value set expressly in a w:p/w:pPr overrides document default pPr, so you'd need to remove the express values to give effect to the document default pPr
Alternatively, you could try setting setParagraphFormatting(FormattingOption.CLASS_TO_STYLE_ONLY), which should result in a Word style matching a class attribute will be used, and nothing else.  In other words, all direct formatting should be ignored.
Or:
Using java Syntax Highlighting
        /**
         * If the CSS white list is non-null,
         * a CSS property will only be honoured if it is on the list.
         * 
         * Useful where suitable default values aren't being provided via
         * @class, or direct values are otherwise providing unwanted results.
         * 
         * Using this should be a last resort.
         * 
         * @param cssWhiteList the cssWhiteList to set
         */
        @Deprecated
        
public static void setCssWhiteList
(Set
<String
> cssWhiteList
) {
                XHTMLImporterImpl.
cssWhiteList = cssWhiteList
;
        }
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