Displaying a database in PDF using iText in Android Studio

I have a function that creates a PDF file that displays a database (SQLite). When I try to open the file, it says: File read error. File type is unsupportted or the file is corrupted.

I don't have any errors in my logcat. I am using iTextg 5.5.3.jar that I added using Sync Project with Gradle Files button of Android Studio.

What am I doing wrong? Please help me.

This is the PDF Create Function:

else if (id == R.id.PDFCreate)
        {
            Cursor cursor = db.rawQuery("SELECT * FROM '" + reciever +"' ",null);
            Cursor title = db.rawQuery("SELECT '" + reciever +"' FROM MasterClasslist", null);

            title.moveToFirst();
            //String filename = title.getString(title.getColumnIndex("Name"));
            String filename = "ClassManager.pdf";

            Document document = new Document();

            document.open();
            File root = new File(Environment.getExternalStorageDirectory(), "Class Manager Grades");

            if (!root.exists())
            {
                root.mkdirs();
            }

            File gpxfile = new File(root, filename);
            try
            {
                PdfWriter.getInstance(document,new FileOutputStream(gpxfile));
            }
            catch (DocumentException e)
            {
                e.printStackTrace();
            }
            catch (FileNotFoundException e)
            {
                e.printStackTrace();
            }

            Paragraph p3 = new Paragraph();
            p3.add("Class Manager");
            try
            {
                document.add(p3);
            }
            catch (DocumentException e)
            {
                e.printStackTrace();
            }

            PdfPTable table = new PdfPTable(6);

            table.addCell("First Name");
            table.addCell("Last Name");
            table.addCell("Prelim Grade");
            table.addCell("Midterm Grade");
            table.addCell("Final Grade");
            table.addCell("Semestral Grade");

            cursor.moveToFirst();
            int count = cursor.getCount();

            for (int j = 0; j < count; j++)
            {
                table.addCell(cursor.getString(cursor.getColumnIndex("FirstName")));
                table.addCell(cursor.getString(cursor.getColumnIndex("LastName")));
                table.addCell(cursor.getString(cursor.getColumnIndex("pGrade")));
                table.addCell(cursor.getString(cursor.getColumnIndex("mGrade")));
                table.addCell(cursor.getString(cursor.getColumnIndex("fGrade")));
                table.addCell(cursor.getString(cursor.getColumnIndex("semGrade")));

                cursor.moveToNext();
            }

            try
            {
                document.add(table);
            }
            catch (DocumentException e)
            {
                e.printStackTrace();
            }
            document.addCreationDate();

            Toast toast = Toast.makeText(getApplicationContext(), "PDF Created", Toast.LENGTH_SHORT);
            toast.show();


        }