Error saving document in SD

I am creating an app and I want to save a report with pdf. From what I have seen the itext library is a good choice. I have the problem when creating the document because it returns an error:

    open failed: EROFS (Read-only file system)

The code I use is:

            Document document = new Document();

            File prueva = new File(getExternalFilesDir(null), "prueva.pdf");
            try {
                PdfWriter.getInstance(document, new FileOutputStream(prueva));
            } catch (DocumentException e) {
                e.printStackTrace();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }


            document.open();

In manifest I have added permissions

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

Any ideas? Thank you very much!