After you select text, a small toolbar appears next to the selection with Copy, Read Aloud, Highlight, Underline, Squiggly and Strike Out. The annotation buttons only appear for documents that support annotations. Turn it off with the Annotations.SelectionToolbar advanced setting. It was ported from the SumatraPDF Plus fork.

PDFs of CAD and engineering drawings are easier to read, especially when zoomed out. Very thin lines get a minimum width that depends on zoom, and pale gray lines are darkened. This happens automatically when a drawing is detected. Control it with the EngineeringDrawingEnhance advanced setting (off / auto / on), or per document with Toggle Engineering Drawing Enhancement (CmdToggleEngineeringDrawingEnhance) in the command palette.