I'm not sure exactly why this happened, but I think I might have some reasons why.
Maybe try changing the color of your text to another color instead of white and see if that works. Or maybe try changing the background from tangerine to something else and see if anything changed once it is converted to a PDF.
Another reason is maybe you did it completely wrong when you converted it. I'll explain how to, incase you completed any steps incorrectly.
1) At the top of your screen, press "File" then "Export". At the top, make sure the "PDF" option is selected and not anything else like "Word" or "ePub" or any of the other options.
2) Select the image quality. Perhaps adjusting this to "Best Quality" rather than just "Good" will help in being able to see the white text. I don't know, just a suggestion.
3) Press "Next" which should be flashing in a light blue color.
4) Save your PDF where you want it and call it what you want.
5) Now press "Export"
6) Open the PDF document and see if anything changed.
Sorry if I haven't been much help but just see if that works.
Another thing I will suggest is to try printing it out and seeing if the text is visible and that might help out.
I'd also suggest, finally, start all over again with converting and follow the steps above, and see if anything changes.
Hope I could help, sorry If I didn't.