How to make it Center

If you center the amounts in the body of the invoice, they will not align correctly for the decimal places. But if you still want it, then amke line 61 of reporting/rep107.php:

    $aligns = array('left',    'left',    'right', 'center', 'right', 'right', 'right');

to be:

    $aligns = array('left',    'left',    'right', 'center', 'right', 'right', 'center');

The top summary table has it's elements already center aligned in the English version - see attachment. The values in it are populated in the $aux_info array in reporting/includes/doctext.inc file.

The file reporting/includes/header2.inc that the invoice report uses aligns the values in it's lines 167-175 ('C'):

        foreach($aux_info as $info_header => $info_content)
        {

            $this->row = $iline2 - $this->lineHeight - 1;
            $this->TextWrap($col, $this->row, $width, $info_header, 'C');
            $this->row = $iline3 - $this->lineHeight - 1;
            $this->TextWrap($col, $this->row, $width, $info_content, 'C');
            $col += $width;
        }

Refer any report where you find a centered field and provide screenshot. We can then trace what works for alignment in Arabic.

Its better you can have this complete header on the top of your invoice. Which will give you better look.

Upload this image jpg to company/0/images/pdfheader.jpg

And refer header2.inc to addimage which will have x and y.  And h and w. Use change that coordinates and make it aligned to fit to your invoice.