The following example, shows how to merge the src.pdf to the end of dst.pdf:

Windows.Storage.StorageFolder localFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
            StorageFile srcFile = await localFolder.GetFileAsync("src.pdf");

            StorageFile dstFile = await localFolder.GetFileAsync("dst.pdf");

            IRandomAccessStream stream1 = await srcFile.OpenAsync(FileAccessMode.ReadWrite);
            IRandomAccessStream stream2 = await dstFile.OpenAsync(FileAccessMode.ReadWrite);

            if (stream1 != null && stream2 != null) {

                PDFDoc docSrc = new PDFDoc();
                PDFDoc docDst = new PDFDoc();

                docSrc.Open(stream1, "");

                docDst.Open(stream2, "");
                docDst.SetCahce(localFolder.Path + "/cache.dat");

                PDFImportCtx ctx = docDst.ImportStart(docSrc);

                int dstno = docDst.PageCount;

                int srccnt = docSrc.PageCount;
                int srcno = 0;

                while (srcno < srccnt) {
                    docDst.ImportPage(ctx, srcno, dstno);

                    dstno++;
                    srcno++;

                }
                docSrc.Close();

                docDst.Save();
                docDst.Close();

            }