Using the PDF engine directly

The components cover signing. For anything else you want to do with a PDF, such as rendering a thumbnail, stamping an image outside a signing session or appending a page of text, the engine underneath is public: SignArgs.Pdfium.PdfDocument, a thin layer over PDFium.

Do not use it to change a document that is being signed: a session stamps and hashes its document itself, exactly once.

Rules

• Stay off the main thread. Every call takes one process-wide lock, because PDFium is not thread-safe. A render can take tens of milliseconds, which is a dropped frame.
• Dispose what you open. A PdfDocument holds native memory. Dispose frees it; the finalizer is only a safety net, and leaks show up as crashes on headsets a few documents later.
• Keep your own bytes. Open copies the bytes it needs to keep, so you may reuse your array.

Render a page

using SignArgs.Pdfium;

var pixels = await Task.Run(() =>
{
    using var document = PdfDocument.Open(bytes);
    var options = new PdfRenderOptions { Dpi = 96, RowOrder = PdfRowOrder.BottomUp };
    var size = document.GetRenderSize(0, options);
    var buffer = new byte[size.Width * size.Height * 4];
    document.RenderPage(0, options, buffer);
    return (buffer, size);
});

var texture = new Texture2D(pixels.size.Width, pixels.size.Height, TextureFormat.RGBA32, false);
texture.SetPixelData(pixels.buffer, 0);
texture.Apply();

Output is RGBA32. PdfRowOrder.BottomUp matches Unity’s texture layout. PdfRenderOptions also takes an exact Width and Height in place of Dpi, and switches for Annotations, FormFields and the Background colour. A second RenderPage overload writes into unmanaged memory with a stride, for reusing one buffer across pages; the engine never allocates the pixels itself.

Read the document

Edit and save

using var document = PdfDocument.Open(bytes);
document.StampImage(0, rgba, width, height, new PdfRect(56, 56, 120, 40));   // rows top-down, straight alpha

var font = PdfFont.Load(File.ReadAllBytes(fontPath));
document.AppendTextPages(new[]
{
    new PdfParagraph("Notes", PdfTextSize.Heading),
    new PdfParagraph("Inspected on site."),
}, new PdfTextPageOptions { Font = font });

byte[] saved = document.Save();

AppendTextPages wraps paragraphs to the margins and adds as many pages as it needs. It follows the same text rules as the audit page: characters the font does not draw are refused, never replaced.

Errors

PdfEngine names the exact PDFium build, as signature records state it.