property SelectionColor: TColor;
Colour drawn over the selected text rectangles. Defaults to clHighlight (the Windows system selection colour, conventionally blue) so the experience matches every other Windows text-editing surface.
Independent from HighlightColor, which colours search-result highlights (default clYellow). The two overlays use different colours and different paint passes — an active search highlight and a live drag-selection can be visible on the same page simultaneously without colour conflict.
begin
PdfView1.SelectionColor := clSkyBlue;
PdfView1.HighlightColor := clYellow;
end;