从ITextSharp迁移至PdfSharp后PDF填充的打印异常与体积问题求助

Let's break down your problems and tackle them step by step—since you've already done great diagnostic work (spotting the /AP element and /NeedAppearance behavior), we can build on that.

Root Causes Recap #

• File Bloat: PdfSharp's default field text setting generates unoptimized /AP (appearance) streams, which Adobe cleans up when you save the file (it compresses streams and removes redundant data).
• Firefox Printing Failure: When /NeedAppearance is present, Firefox doesn't properly render the field's appearance (unlike Adobe, which generates a correct /AP on the fly). Removing /NeedAppearance makes other readers match Firefox's broken output because they can't fall back to generating a proper appearance.

Fix 1: Generate Proper, Optimized /AP Elements Manually #

PdfSharp's PdfTextField.Text property creates a basic appearance that doesn't handle multi-line text or font consistency well. We need to replace this with a custom appearance generator that mimics Adobe's behavior.

Here's a modified SetField method that generates a proper multi-line appearance with consistent font:

public static bool SetFieldWithProperAppearance(this PdfAcroForm form, string fieldName, string value)
{
    if (form.Fields.TryGetValue(fieldName, out PdfField rawField) && rawField is PdfTextField field)
    {
        // Get the field's default font (fall back to Helvetica if none is set)
        PdfFont font = field.Elements.GetValueOrDefault(PdfAnnotation.Keys.DA) is PdfString daString 
            ? PdfFont.FromDescriptorString(daString.Value, form.Document) 
            : new PdfFont(form.Document, PdfFontFamily.Helvetica, 10);

        // Get the field's bounding box
        PdfRectangle rect = field.Rectangle;

        // Create a new appearance stream
        PdfFormXObject appearance = new PdfFormXObject(form.Document);
        appearance.Size = rect.Size;

        using (XGraphics gfx = XGraphics.FromPdfForm(appearance))
        {
            // Set up text formatting for multi-line support
            XStringFormat format = new XStringFormat
            {
                Alignment = XStringAlignment.Near,
                LineAlignment = XLineAlignment.Near,
                Trimming = XStringTrimming.None,
                FormatFlags = XStringFormatFlags.LineLimit
            };

            // Draw the text within the field's bounds, allowing wrapping
            gfx.DrawString(value, font, XBrushes.Black, 
                new XRect(0, 0, rect.Width, rect.Height), format);
        }

        // Assign the custom appearance to both normal and down states
        field.Elements.SetValue("/AP", new PdfDictionary
        {
            { "/N", appearance }
        });

        // Update the field's actual value
        field.Text = value;

        return true;
    }
    return false;
}

Fix 2: Optimize File Size After Flattening #

After setting fields and flattening, we need to mimic Adobe's stream optimization. Add this method to compress redundant streams and clean up unused objects:

public static void OptimizePdfSize(PdfDocument document)
{
    // Compress all streams
    foreach (PdfObject obj in document.Objects)
    {
        if (obj is PdfStream stream)
        {
            stream.Compress();
        }
    }

    // Remove unused objects (PdfSharp might leave some behind)
    document.RemoveUnusedObjects();
}

Fix 3: Adjust /NeedAppearance Logic #

Once we've generated proper /AP elements, we can safely handle the /NeedAppearance property:

1. After setting all fields with the custom appearance method, remove the /NeedAppearance property from the AcroForm. This tells all readers to use the pre-generated appearance instead of trying to create their own.
2. Skip this step only if you need readers to dynamically regenerate appearances (but in your case, this is causing the Firefox issue).

Add this line after setting all fields:

form.Elements.Remove("/NeedAppearance");

Full Workflow #

Your updated code should look like this:

// Set fields with custom appearance
form.SetFieldWithProperAppearance("YourFieldName", "Multi-line\nText Example");

// Remove NeedAppearance since we have valid appearances
form.Elements.Remove("/NeedAppearance");

// Flatten the document
document.Flatten();

// Optimize file size
OptimizePdfSize(document);

// Save the document
document.Save("Output.pdf");

Why This Works #

• The custom appearance generator ensures multi-line text wraps correctly and uses the field's specified font, fixing the Firefox printing issue.
• Optimizing streams and removing unused objects reduces the file size to match what Adobe produces.
• Removing /NeedAppearance ensures all readers use our pre-generated appearance, eliminating inconsistencies between viewers.

内容的提问来源于stack exchange，提问作者Mikkel_Gundersen