Apple Mail - PDF Form appears blank

Hi, can't seem to find any other posts on this issue.

I have an IT Request PDF form created in Acrobat.  Once the user fills it in, it gets sent to the director for approval.  The Director uses an iPad Pro with the latest iOS and Apple Mail.  When he opens the PDF in Mail it just displays blank.  If he then opens in a PDF app it's fine.

The reason I'm looking for a fix is that he wants to use the Apple Markup & Review to sign off the request as this makes it really simple. 

Anyone else come across this and got a solution?

Thanks in advance,

Fiona