Body text fade from dark to lite

[BenJolivet]

When I export I finished book to PDF, the body text fades from darker, thicker strokes to lite-weight strokes and I can't figure out why. 

[Andy05]

Maybe it would be good if you could share (a part of) the document. Because right now, it's just a stab in the dark from me. This problem might occur, if some transparency effect is under- or overlaying parts of the text. 

[GarryP]

Just out of interest I extracted all of the “t”s from the penultimate line and arranged them together – see first attached image (right-hand example is a duplicate of the left-hand one, for comparison).
There does seem to be some marked differences along the line but the differences don’t look ‘regular’ to me.
I was able to reproduce the same sort of thing, as mentioned by Andy05 above, with a Gradient Fill on the Stroke which contained a transparency on the right – see second attached image.
If this isn’t what has happened then we may need to see the actual document (or a cut-down version).
P.S. Check the exported document with some other PDF readers to see if there is a difference. It might be the PDF reader that is to blame.

[Andy05]

20 minutes ago, GarryP said:

I was able to reproduce the same sort of thing, as mentioned by Andy05 above, with a Gradient Fill on the Stroke which contained a transparency on the right – see second attached image.

Sometimes even transparent areas of an overlapping PNG can cause the same unwanted effects.