public partial class Form1 : Form
    {
        public Form1()
        {
            PdfCommon.Initialize("");
            InitializeComponent();
        }

        private void BtnLoad_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog ofd = new OpenFileDialog
            {
                AutoUpgradeEnabled = true,
                CheckFileExists = true,
                CheckPathExists = true,
                ReadOnlyChecked = true,
                DefaultExt = "pdf",
                DereferenceLinks = true,
                SupportMultiDottedExtensions = true,
                ShowHelp = false,
                Multiselect = false,
                ShowReadOnly = false,
                Title = "Please select a PDF file."
            })
            {
                DialogResult dr = ofd.ShowDialog();
                if ((dr == DialogResult.Yes) || (dr == DialogResult.OK))
                {
                    PdfDocument doc = PdfDocument.Load(ofd.FileName, new PdfForms());
                    doc.Pages[0].PageObjects.OfType<PdfTextObject>().First().FontSize += 20;
                    doc.Save(@"C:\temp\Output.pdf", SaveFlags.RemoveUnusedObjects);
                }
            }
        }
    }