This will have little if nothing to do with TinyMCE as you need to tell MPDF to set the margins on the page.
You can specify margins in millimetres in the MPDF constructor.
$margin_left
, etc:
Sets the page margins for the new document. All values should be specified as LENGTH in millimetres. If you are creating a DOUBLE-SIDED document, the margin values specified will be used for ODD pages; left and right margins will be mirrored for EVEN pages. BLANK or omitted uses the default values.