empty object; closure issue? Node > Sqlite > File > PDF

I would like the below to read image files and turn them into a PDF. The files are stored on the disk, and the filename are stored in a SQLite database.

var fs = require('fs');
var pdf_doc = new jsPDF();

pdf_doc.text(20, 20, 'Hello, world.');
db.all("SELECT * FROM media", function(err, rows) {
    rows.forEach(function(row) {
        var file_content = read_image_file(row.url);
        add_image_to_pdf_doc(file_content);
    });
});

function read_image_file(image_url) {
    var img_data = fs.readFileSync("./public/uploads/"+image_url);
    return img_data;
}
function add_image_to_pdf_doc(img_data) {
    pdf_doc.addImage(img_data, 'JPEG', 15, 40, 180, 160);
}

pdf_doc seems to be empty ("Uncaught TypeError: undefined is not a function") and I can't get my head around why nor how to solve this.