It seems that you are using Segoe UI Semilight and then applying the Bold command to it in MS Word. What happens is that MS Word applies algorithmic (synthetic) bolding, by thickening the strokes of letters. Such bolding is lost when converting to PDF.
The solution is to use, for each font family, only the font weights that are actually available as typofaces designed by a typographer. For the Segoe UI family, these are Semilight, Regular (normal), Semibold, and Bold – more than you usually have in fonts that you get with a computer (as opposite to professional fonts that you buy).
MS Word has a little odd system where the Bold command either selects a bold typeface or performs algorithmic bolding. The way to get Segoe UI Bold is to set the font to Segoe UI, then use the Bold command (or select a style that uses it), whereas Segoe UI Semilight and Segoe UI Semibold need to be selected in the Font menu.
What you probably want is Semibold, as that is what algorithmically bolded Segoe UI Semilight resembles.