Problem displaying "closed" letters

I have a document which displays perfectly in Adobe Reader in Windows 10.  I transfer it by Dropbox to an Ipad and the display is weird in the following respects:

all "closed" letters have the closed bit filled black - eg top triangle In A, all of D etc.  Not a major problem - but does anyone know why this is happening and if there is a fix?  I think it has also happened in the past when printing.

Thanks in advance for any help>

John