This article shows you how to add attachment.

You need a professional or premium license.

Just add this method in PDFView class:

- (void)addAttachmentAtPage:(int)pageno fromPath:(NSString *)path inRect:(PDF_RECT)rect
{
    PDFPage *page = [m_doc page:pageno];
    [page addAnnotAttachment:path :0 :&rect];
    [m_doc save];
}