A text width also depends on a particular font used.
By default (when no TTF embedding configured), PD4ML outputs PDFs refer to Acroread’s built-in Type1 fonts; a Web browser most probably uses standard TTF from OS.
Helvetica of Acroread looks very similar to Arial, but in fact they are not exactly the same (especially in glyph widths). Probably that is the reason.
Try to configure TTF embedding and compare PDF and rendered HTML again.