function CharacterIndexAtPos(X, Y: Integer; ToleranceX, ToleranceY: Double): Integer;
Returns the character index in the current page nearest to the screen pixel (X, Y), or -1 if no character is close enough.
Use in OnMouseDown to initiate text selection.
TPdfView maintains the device-to-page transform internally based on Zoom, Rotation, page padding, and scroll offset; these helpers are the safe public interface to that transform.
PageNumber := assignments invalidate cached coordinates.Rotation, so a page rotated 90° still maps to the correct PDF rectangle.
procedure TForm1.PdfView1MouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
PageX, PageY: Double;
begin
if PdfView1.CharacterIndexAtPos(X, Y, PageX, PageY) then
Caption := Format('pt = (%.2f, %.2f)', [PageX, PageY]);
end;