First, place the file in assets/pdf/ folder. Then, use the following html in the markdown post:

<object data="{{ site.url }}{{ site.baseurl }}/assets/pdf/FILENAME.pdf" width="100%" height="1000" type="application/pdf"></object>

Alternatively, you can simply provide the link, using:

[link name](assets/pdf/FILENAME.pdf)

Attention is All You Need

For the link to open in new tab / window, use html syntax to set the target="_blank".

<a href="{{ site.url }}{{ site.baseurl }}/assets/FILENAME" target="_blank">Yay</a>

Marp Presentation