pdf.Security.DecryptOwnerPassWord(string ownerPassword);
pdf.Security.DecryptUserPassWord();

PdfDocument doc = new PdfDocument();
doc.LoadFromFile(input);
int pageIndex = GetAttachmentPage(doc.Pages, attachmentName);

private int GetAttachmentPage(Spire.Pdf.Widget.PdfPageCollection pageCollection, string fileName)
{
    if (string.IsNullOrEmpty(fileName))
    {
        throw new ArgumentException("fileName is null or empty");
    }
    for (int i = 0; i < pageCollection.Count; i++)
    {
        PdfPageBase page = pageCollection[i];
        for (int j = 0; j < page.AnnotationsWidget.Count; j++)
        {
            Spire.Pdf.Annotations.PdfFileLinkAnnotationWidget pdfFileLinkAnnotationWidget = page.AnnotationsWidget[j] as Spire.Pdf.Annotations.PdfFileLinkAnnotationWidget;
            if (pdfFileLinkAnnotationWidget != null)
            {
                if (fileName.Equals(pdfFileLinkAnnotationWidget.FileName))
                {
                    return i;
                }
            }
        }
    }
    return -1;
}