Check your file -> print menu   See if that is setup now in portrait then try and change it.   The browser is typically controlled by Print setting setup so try changing it then try resending the report.

what if you try from an incognito window?

Great point Jen...   Sophie if you want to see if your base report is too big try doing a control p of the browser with dashbaord on it see what sort of scaling is required to make it fit.   That could help you scale width down to fit more in line with an A4/letter size.

You are using send file on schedule right?   The pcs you use are they using any VPN to access the scheduling reports? 

ok no worries this is good as part of troubleshooting... Know Im asking alot but, can you try from a different PC?   I suspect its your windows printer settings may have some sort of auto size command...   Be great to rule out any PC setting this way.

Can you try a different browser to send the email?

What language is your PC set to?   any way to make a screen grab of some of the weird areas?

what operating system and browser are you using to create the PDF and setup schedule?

You may also want to look at activity map tracking.   1 no extra dev work out of box 2 can allow you to do advanced mappings links/regions reporting 3 allows you to due near realtime (basic heatmaps) on page vis browser extension   Then just use custom links when coding/pae doesnt auto track activit...

This is not unusual and sadly more common than people realize. I like to call it a sort of click fraud.   You mention its not about ads so its important to note are the links clicked truly viewed by user links or links multi scrolls down? Make sure you tag each CTA relevantly to allow you to differe...