This article shows how to use OpenMem method to open pdf with data (note: the pdf will be readonly).

int it;
for(it = 0; it < 512; it++) {
    try {
        InputStream str = getAssets().open("test.PDF");
        byte data[] = new byte[115557];
        str.read(data);
        Document doc1 = new Document();
        doc1.OpenMem(data, null);
        doc1.Close();
        str.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}