We apologize for the delay in response to your query.
Are you using Adobe Acrobat Reader on desktop as well as on mobile to open or work on pdf files?
Also, the pdf file that you are referring here, is that pdf file is created using Adobe's Acrobat application or any other pdf maker? It could or could not be issue with the font used to create the pdf file, which isn't displaying correctly when opening in mobile app.
Please check with a different pdf file and see if that is also changing the fonts when you open it on mobile device.