Font substitution from Mac to PC when reading a EPUB (Fixed Layout) file

Hi,

I created a Fixed Layout EPUB document using InDesign CC 2017 (Mac). When I open the EPUB document in both Adobe Digital Editions and iBooks it's great, the fonts works, images all in the correct places etc. When I open the document on my PC in Adobe Digital Editions 4,5,4 for Windows, the images etc are all fine and in the right places, but the fonts default to Arial and Times, the fonts used in the Mac document are Gills Sans and Baskerville (OpenType), why is this, can anybody help?