static void Main(string[] args)
{
    using (var doc = PdfDocument.Load(@"test.pdf"))
    {
        ExtractAttachmentsFromCatalog(doc);
        ExtractAttachmentsFromAnnotations(doc);
    }
}

private static void ExtractAttachmentsFromCatalog(PdfDocument doc)
{
    if (doc.Root.ContainsKey("Names"))
    {
        var names = doc.Root["Names"].As<PdfTypeDictionary>();
        if (names.ContainsKey("EmbeddedFiles"))
        {
            var embeddedFiles = names["EmbeddedFiles"].As<PdfTypeDictionary>();
            var attachments = embeddedFiles["Names"].As<PdfTypeArray>();
            for (int i = 0; i < attachments.Count; i += 2)
            {
                string attachmentName = attachments[i].As<PdfTypeString>().UnicodeString;
                var attachmentDict = attachments[i + 1].As<PdfTypeDictionary>();
                var attachmentStream = attachmentDict["EF"].As<PdfTypeDictionary>()["F"].As<PdfTypeStream>();
                var data = attachmentStream.DecodedData;
                WriteExtractedData(attachmentName, data);
            }
        }
    }
}

private static void ExtractAttachmentsFromAnnotations(PdfDocument doc)
{
    foreach (var page in doc.Pages)
    {
        if (page.Annots == null)
            continue;
        foreach (var annot in page.Annots)
        {
            if (annot is PdfFileAttachmentAnnotation)
            {
                var fileSpec = (annot as PdfFileAttachmentAnnotation).FileSpecification;
                var file = fileSpec.EmbeddedFile;
                byte[] fileContent = file.Stream.DecodedData;
                WriteExtractedData(fileSpec.FileName, fileContent);
            }
        }
        page.Dispose();
    }
}

private static void WriteExtractedData(string name, byte[] data)
{
    System.IO.File.WriteAllBytes($@"Out\{name}", data);
}