This is a guess which should be only a comment- we know nothing of the construction of your document. We know as little of the used PDF variation.
You would have a possibility to get a proper answer if you made available for ex a simplified version of the Ai file and the PDF, something like this:
or if you revealed the details otherwise.
I guess you have blended between 2 fully opaque white non-filled outlines of a letter. The foreground shape has reasonably thick white stroke and the background shape in the blend has zero width stroke or no stroke at all. Illustrator renders it except some Moire can be seen on the screen. In PDF too thin outlines are changed to a certain minimum width.
Fixing: Use constant width strokes, blend from fully opaque white to fully opaque black.
In Acrobat the blend from white to black with constant stroke width looks the same as in Illustrator, but the blend from white to white with zero thickness looks in Acrobat white from the start to the end and in small size there's also very disturbing Moire.
Opening the PDF again in Illustrator showed it's good, only Acrobat had replaced too thin lines with thicker ones. But that doesn't help at all when you must assure the customer. In addition the PDF was the default one with zero settings for any certain print process.
It's another question what the print process thinks and does it also try to make it "better" like Acrobat did. Demand a soft proof to see. Start by changing the blends as suggested - have at least one trap less.
BTW. If you blend from white to black you can make the brightness change steeper by setting to the blending mode of the blending result to Hard Light in the transparency panel (only an option, maybe not wanted).