Really lazy to use Microsoft Word to write something. So I proceed to try pandoc with Miktex installed on Windows.

Finally, got what I need and I have to start to think how to write the letter...

Final result

Based on the template from Pandoc Letter Template (DIN 5008), with the following modifications:

everything left align
show page number at every page

Refer to the gist for the template:

letter-template.latex

The Process

Try 2

https://github.com/mrzool/letter-boilerplate

Try 3

https://github.com/aaronwolen/pandoc-letter

Try 4

Pandoc Letter Template (DIN 5008)

Try 4 looks the best so far. And if you want to align everything to left, just need to do the following:

Command

pandoc --template=letter.latex example\letter.md -o output.pdf

From

\documentclass[
    foldmarks=true,      % print foldmarks
    foldmarks=BTm,       % show foldmarks top, middle, bottom
    fromalign=right,     % letter head on the right
    fromphone,           % show phone number
    fromemail,           % show email
    fromlogo,            % show logo in letter head
    version=last        % latest version of KOMA letter
]{scrlttr2}

To

\documentclass[
    foldmarks=true,      % print foldmarks
    foldmarks=BTm,       % show foldmarks top, middle, bottom
    fromalign=left,     % letter head on the right
    fromphone,           % show phone number
    fromemail,           % show email
    fromlogo,            % show logo in letter head
    version=last,        % latest version of KOMA letter
	refline=dateleft     % aligns both the city and the date to the left
]{scrlttr2}