How do you draw a straight underline in Preview?

As I mentioned, that fuzzy line calls for a change to your line style, where you will find varying thickness of solid lines.

Hint: Markup > Shape styles, per my original reply to you. If the paint brush "fuzzy" line is selected, then click it again on the menu to deselect it.

For all the varied thickness choices, the lines are not really solid, they look somewhat like drawn with a crayon - that is the point that the OP is making. As far as I can tell, there is no way to change that.

Here are lines drawn with varying thicknesses:

And with the paint brush line style deselected and at 150%, I cannot reproduce the line fuzziness with Preview on macOS 13.2.1. The following is as good as it is going to get in this version of Preview.

Using Markup > Shapes, simply clicking the line segment icon drops a straight line into the PDF. Alternatively, you can press the command key while selecting the Sketch tool and any wiggly drawn line segment will snap to a straight line. In either case, choose the line thickness and color from the respective shape styles and color controls.

We are fellow users and other than the occasional hosting team community specialist, Apple and the product teams are not here.

To underline text, you can select it and press Command-Control-u

Quick and easy.

Or if you otherwise want a straight line press Command-control-i

As VikingOSX pointed out, you can adjust this line and if you hold down the shift key it will only reorient in 45-degree increments.

Oh, I see now. It is not a matter of being horizontal, but rather having a solid line rather than a "crayon-like" line.

I don't see any options to change that. This changed some time ago (maybe in Big Sur, I can't say for sure), it is not new in Ventura.

All any of us can do is use feedback and request that feature. It should be a simple menu choice, and I agree that it should be there (though, as a teacher annotation my students' submissions, I actually like the current form).

https://www.apple.com/feedback

You can easily add vertical lines, as I mentioned before.

Command-control-I, then hold down Shift and rotate the end of the line; it will move 45 degrees at a time.

Keep holding down Shift to drag and extend without changing the line orientation.

Option-drag to make a copy.

As I said, it currently does not seem to be possible to have a full solid line.

[Redacted]

I stand corrected!