Automated PDF Preflight and Risk Auditing with PDFium

In enterprise printing, archiving, and document compliance workflows, a PDF file cannot simply be "rendered." It must be audited. A PDF might contain unflattened transparency groups that crash legacy raster image processors (RIPs), embedded JavaScript that poses a security risk, or low-resolution images that will look terrible when printed at 300 DPI.

This process of inspecting a PDF before it enters a production workflow is known as Preflighting. In this article, we will explore how to build an automated preflight and risk auditing tool in Delphi by tapping into the low-level parsing capabilities of PDFium.

Key Preflight Checks

A standard risk audit typically checks for the following elements within a PDF:

• Interactive Elements: JavaScript actions, Launch actions (executing external programs), and AcroForms.
• Resource Metrics: Presence of low-resolution images or missing embedded fonts.
• Security Status: Document encryption, password requirements, and access permissions (e.g., printing disabled).
• Document Standards: Validation of PDF/A or PDF/X conformance markers in the document metadata.

Extracting PDF Metadata and Annotations with PDFium

PDFium provides a robust C API that Delphi can consume. To perform a preflight audit, we don't just render the page; we walk the PDF object tree. Let's look at how to iterate through the document pages and inspect annotations to find potentially risky JavaScript actions.

uses
  System.SysUtils, pdfium_lib;

procedure AuditPdfSecurity(const FileName: string);
var
  Doc: FPDF_DOCUMENT;
  PageCount, i, j: Integer;
  Page: FPDF_PAGE;
  AnnotCount: Integer;
  Annot: FPDF_ANNOTATION;
  AnnotSubType: FPDF_ANNOTATION_SUBTYPE;
  Action: FPDF_ACTION;
  ActionType: ULONG;
begin
  FPDF_InitLibrary();
  try
    // Load the document without a password
    Doc := FPDF_LoadDocument(PAnsiChar(AnsiString(FileName)), nil);
    if Doc = nil then
      raise Exception.Create('Failed to load document or password required.');
      
    try
      PageCount := FPDF_GetPageCount(Doc);
      Writeln(Format('Auditing %d pages...', [PageCount]));
      
      for i := 0 to PageCount - 1 do
      begin
        Page := FPDF_LoadPage(Doc, i);
        if Page <> nil then
        begin
          AnnotCount := FPDFPage_GetAnnotCount(Page);
          for j := 0 to AnnotCount - 1 do
          begin
            Annot := FPDFPage_GetAnnot(Page, j);
            if Annot <> nil then
            begin
              AnnotSubType := FPDFAnnot_GetSubtype(Annot);
              
              // Check for Link Annotations that might have malicious actions
              if AnnotSubType = FPDF_ANNOT_LINK then
              begin
                Action := FPDFAnnot_GetLinkAction(Annot);
                if Action <> nil then
                begin
                  ActionType := FPDFAction_GetType(Action);
                  // 2 represents PDFACTION_URI, 3 represents PDFACTION_SOUND, 4 represents PDFACTION_MOVIE
                  // We specifically look out for PDFACTION_LAUNCH or PDFACTION_JAVA_SCRIPT
                  if (ActionType = PDFACTION_JAVA_SCRIPT) or (ActionType = PDFACTION_LAUNCH) then
                  begin
                    Writeln(Format('WARNING: Malicious action detected on page %d', [i + 1]));
                  end;
                end;
              end;
              FPDFPage_CloseAnnot(Annot);
            end;
          end;
          FPDF_ClosePage(Page);
        end;
      end;
    finally
      FPDF_CloseDocument(Doc);
    end;
  finally
    FPDF_DestroyLibrary();
  end;
end;

Checking Image Resolution

Another critical preflight step for print workflows is ensuring no image falls below a specific DPI threshold. PDFium allows you to extract image objects directly from the page stream. By dividing the pixel dimensions of the extracted image by the physical dimensions (in points) it occupies on the PDF page, you can calculate the effective DPI.

procedure AuditPageImages(Page: FPDF_PAGE);
var
  ObjCount, i: Integer;
  PageObj: FPDF_PAGEOBJECT;
  ImgWidth, ImgHeight: Integer;
begin
  ObjCount := FPDFPage_CountObjects(Page);
  for i := 0 to ObjCount - 1 do
  begin
    PageObj := FPDFPage_GetObject(Page, i);
    if FPDFPageObj_GetType(PageObj) = FPDF_PAGEOBJ_IMAGE then
    begin
      FPDFImageObj_GetBitmap(PageObj); // Returns an FPDF_BITMAP you can inspect
      // Retrieve metadata using FPDFImageObj_GetImageMetadata
      // Calculate effective DPI based on quad coordinates
    end;
  end;
end;

Integrating with PDFium Component

Building a robust preflight engine from scratch using the raw PDFium C API requires significant boilerplate code and deep knowledge of the PDF specification. Using a wrapper simplifies this immensely. With a high-level Delphi wrapper, you can turn complex C pointer iterations into clean object-oriented code.

By automating the preflight phase, you prevent bad files from gumming up your production pipeline, saving both time and rendering resources.

Note: Advanced PDF parsing and preflight capabilities are fully supported by the PDFium Component.