FPDF_InitLibrary();
FPDF_STRING test_doc = "myTest.pdf";
FPDF_DOCUMENT doc = FPDF_LoadDocument(test_doc, NULL);
if (!doc) {
    qWarning() << " error loading PDF file";
    return 1;
}
FPDF_PAGE page = FPDF_LoadPage(doc, 0);
if (!page) {
    qWarning() << " error loading first page";
    return 1;
}

FPDF_ANNOTATION highlightAnnot = FPDFPage_CreateAnnot(page, FPDF_ANNOT_HIGHLIGHT);

if (highlightAnnot) {
    // Set the annotation rectangle.
    FS_RECTF rect;
    rect.left = 72;
    rect.bottom = 504;
    rect.right = 132;
    rect.top = 520;
    if(!FPDFAnnot_SetRect(highlightAnnot, &rect)) {
        qWarning() << " error in FPDFAnnot_SetRect()";
    }
    // When using AP this function should not be used. See doc in FPDF_annot.h
    if(!FPDFAnnot_SetColor(highlightAnnot, FPDFANNOT_COLORTYPE_Color, 255, 255, 0, 255)) {
        qWarning() << " error in FPDFAnnot_SetColor()";
    }
    // Append a new set of quadpoints.
    FS_QUADPOINTSF new_quadpoints;
    new_quadpoints.x1 = 72.f;
    new_quadpoints.y1 = 520.f;
    new_quadpoints.x2 = 132.f;
    new_quadpoints.y2 = 520.f;
    new_quadpoints.x3 = 72.f;
    new_quadpoints.y3 = 504.f;
    new_quadpoints.x4 = 132.f;
    new_quadpoints.y4 = 504.f;
    if(!FPDFAnnot_AppendAttachmentPoints(highlightAnnot, &new_quadpoints)) {
        qWarning() << " error FPDFAnnot_AppendAttachmentPoints()";
    }
}
FPDF_ClosePage(page);

// writing file with FPDF_SaveAsCopy()
...
FPDF_DestroyLibrary();
return retCode;