Fillable forms

Form controls become fillable PDF form fields (AcroForm), drawn as Chrome prints them and filled with their current values. Adobe Reader, Preview, Chrome, Firefox and other viewers let the reader fill them in and save the result. The fields of an existing PDF can be read, filled and flattened too.

var html = """
    <p>Name <input name="name" value="Jane Doe" required></p>
    <p><label><input type="checkbox" name="newsletter" checked> Send me the newsletter</label></p>
    <p><input type="radio" name="size" value="S"> S <input type="radio" name="size" value="M" checked> M</p>
    <p><select name="country"><option value="dk">Denmark</option><option value="se">Sweden</option></select></p>
    <p><textarea name="comments" rows="4" cols="40"></textarea></p>
    """;
byte[] pdf = HtmlToPdf.Convert(html);

// Or print the controls as they are, without form fields:
byte[] flat = HtmlToPdf.Convert(html, new PdfOptions { FormFields = false });

HTML	PDF field
`<input>` of type `text`, `email`, `tel`, `url`, `search`, `number`, `date` and the like	Text field (`maxlength` sets the maximum length)
`<input type="password">`	Password text field; the value is drawn as dots and not stored
`<textarea>`	Multi-line text field
`<input type="checkbox">`	Check box; its `value` is the export value (default `Yes`)
`<input type="radio">`	Radio buttons: those with the same `name` are one field, and each `value` is a choice
`<select>`	Drop-down list (combo box); with `multiple` or `size` > 1 a list box, multi-select with `multiple`
`<button>`, `<input type="submit">`, `reset`, `button`	Drawn only: a PDF has nothing to submit

Field names and behaviour

Fields are named after the name attribute (a period becomes _, since PDF uses it to nest fields; a repeated name gets _2, _3, ...; a control without a name gets text1, checkbox1, ...). readonly and required set the field's flags, title, aria-label or its <label> becomes its tooltip, and a placeholder is drawn in grey while the field is empty. Disabled controls (or controls in a disabled <fieldset>) are drawn but not fillable, and hidden inputs are left out. Each field has its own appearance, so the PDF looks right before anyone touches it and in viewers that never redraw fields; while a field is edited, the viewer draws its text in Helvetica, Courier or Times at the control's size and colour. Forms work with Security (the values are encrypted with the rest), Conformance (veraPDF passes the fields as PDF/A-2b and 3b), Accessibility (each widget sits in a Form element described by its label, title or name, and veraPDF passes PDF/UA-1), Signature (the signature field joins the form) and PdfMerger (repeated names are renamed).

Filling and flattening existing PDFs

PdfForm reads, fills and flattens the form fields of any PDF, not only those PdfPecker made, such as a government form or a contract template from another tool:

// The fields, by fully qualified name ("address.city" for a nested one).
foreach (PdfFormField field in PdfForm.GetFields(pdf))
    Console.WriteLine($"{field.Name} ({field.Type}) = {field.Value}, choices: {string.Join(", ", field.Options)}");

var values = new Dictionary<string, string>
{
    ["name"] = "Jane Doe",
    ["address.city"] = "Copenhagen",
    ["newsletter"] = "true",       // a check box: its on state, or true/yes/on/1; Off/false/no/0/"" clears it
    ["size"] = "M",                // a radio button group: the state name of one button
    ["country"] = "dk",            // a combo box or list box: a choice's value or its text
};
byte[] filled = PdfForm.Fill(pdf, values);

// Fill and flatten in one go, so the values can no longer be changed; or flatten a PDF as it is.
byte[] final = PdfForm.Fill(pdf, values, new PdfFormFillOptions { Flatten = true });
byte[] flat = PdfForm.Flatten(filled);
PdfForm.FillFile("application.pdf", "application-filled.pdf", values);

PdfFormField gives each field's Type (Text, CheckBox, RadioButton, ComboBox, ListBox, PushButton or Signature), its Value (and Values for a list box with several choices), the Options it accepts, its flags (ReadOnly, Required, Multiline, MaxLength, ...), its tooltip and the page it is on. A name that is no field, a choice the field does not have, or text longer than MaxLength throws ArgumentException; set IgnoreUnknownFields to skip names the PDF does not have. Push buttons and signature fields cannot be filled (sign the latter with PdfSigner).

Every filled field gets a new appearance in the field's own font, size, colour and alignment, with automatic font size, multi-line text and comb fields laid out, and the box and background of its old appearance kept, so the values show in every viewer and in PdfToImage. When the field's font cannot show a value, a subset of PdfFormFillOptions.Font (by default the bundled Liberation Sans, which covers Latin, Greek and Cyrillic) is embedded for it. XFA form data, which would override the filled fields in some viewers, is removed.

A signed or encrypted PDF is filled with an incremental update: the original bytes stay as they were, so the signatures stay valid (when their permissions allow form filling) and the encryption is kept; pass the password in PdfFormFillOptions.Password. Other PDFs are written out in full, without the old values in earlier revisions. Flattening draws each field into its page and removes it, keeping links, comments and other annotations; it changes the pages, so a signed PDF is refused with InvalidOperationException.