Stroke transparency

If Steve's answer isn't completely clear, there is a great deal of variation between PDF viewers. The absolute one is always Adobe Acrobat Reader, of course, and there are good aftermarket/third party ones. However many of those implement variations and shortcuts that mean "fancy" elements such as selective transparency might not be handled well, at all, or the same way Acrobat Reader does it.

And the PDF viewers built into web browsers are, for the most part, convenience utilities with very limited rendering range. They are meant for viewing simple PDF documents with ease, not being fully up to all PDF abilities such as transparency, interactive controls, etc.

If (as Steve notes) your audience is web users, you will have to work to the limitations of the common web readers. If you want 'perfect' performance, you will have to limit or direct your readers to using genuine Acrobat Reader.

@Mii2748990340p2 said: "It only happens around punctuation marks."

Did you do three individual dots next to each other?

If yes, try to use a different glyph instead, a HORIZONTAL ELLIPSIS, Unicode 2026. Can be found in the Glyphs panel.

Or could be typed with keyboard shortcut Alt + . on a Mac or with Alt + 133 (on the number pad) of a Windows machine.

Hope, that your font has it.

Regards,
Uwe Laubender
( Adobe Community Expert )

How are you going to use it? Is it going to be printed? If so, what shows in Acrobat should show it more accurately that browser PDF viewers.

If it's for web use only, then how it looks in a browser would be most important.

What's happening in the example isn't really easy to see. It all just looks kind of messy. A more demonstrative example might help. But in any case, I'm not really sure what to say about this; applying a stroke to live text is often a risky endeavor that can produce varied results depending on the size of the type, the size of the stroke, the integrity of the font, etc.