Highlighting, underlining and strikethrough

The easiest way: select the text and tap Highlighter / Underline / Strikethrough in the selection menu. If you'll make lots of marks, choose the tool and colour (Yellow, Green, Blue, Pink, Orange, Purple, Red, Black) from Highlight in Markup mode; from then on, running your finger over the text marks it directly; switch back to Select when you're done. Marks only sit on the letters; gaps between tables and columns aren't painted. Marks are stored with the document and listed with page numbers in the Notes panel.

Tap marked text and a frame and a small action card open: change the mark type, Add comment / Edit comment, Copy, Read aloud, Delete.

Highlights, underlines/strikethroughs, drawings and sticky notes made in another app (Adobe, Google PDF, Samsung, a browser…) and saved inside the PDF become marks when the document first opens: they show, can be deleted or recoloured, and are listed in the Notes panel. If you delete them and choose "Save as PDF", they are removed from the file too.