Conversion Has Dark Patches for images..

I am using a win10 pc.

I have an epub document which I asked calibre to convert to pdf for me and it did the operation without any apparent problem.

But when I come to view it I see that the titles are there plain to see but the 'chapter' content is just a page sized dark area.

It is a songbook. the 'titles' are the names of the songs. What I'm calling 'the chapters' would be the actual manuscript music for the song.

I have had this kind of thing before: pdf files which acrobat and other things cannot open without these missing black areas.

I used to think they were corrupt files but have since found a prog which will open them every time without a problem - displaying the missing 'black' content.

I can try that prog on this .pdf file if I know how to get the file out of calibre and into another dir somewhere?

And any ideas of what's wrong?