Modify Existing PDFs

Use createWriterToModify to add pages or content to an existing PDF. For a separate output file, provide modifiedFilePath in the options object.

var writer = muhammara.createWriterToModify("input.pdf", {
  modifiedFilePath: "output.pdf",
});

To add content to an existing page, use PDFPageModifier, start its context, draw content, end the context, and write the page. Passing true as the third constructor argument isolates new graphics from the existing graphics state.

Replace an Indirect Object

replaceObject(pageIndex, sourceObjectId, replacementObjectId, options) rewrites a page dictionary so every direct reference to sourceObjectId points at replacementObjectId instead. pageIndex is zero-based, and the method is available only on a writer created with createWriterToModify.

var reader = muhammara.createReader("input.pdf");
var contentsId = reader
  .parsePage(0)
  .getDictionary()
  .queryObject("Contents")
  .toPDFIndirectObjectReference()
  .getObjectID();

reader.end();

var writer = muhammara.createWriterToModify("input.pdf", {
  modifiedFilePath: "output.pdf",
});
var objectsContext = writer.getObjectsContext();
var replacementId = objectsContext.startNewIndirectObject();
var replacement = objectsContext.startPDFStream();

replacement.getWriteStream().write(Buffer.from("BT ET"));
objectsContext.endPDFStream(replacement).endIndirectObject();
writer.replaceObject(0, contentsId, replacementId);

Only the named page is rewritten, so an object shared by several pages keeps its old reference elsewhere. Pass { scope: "global" } to apply the same replacement to every page instead:

writer.replaceObject(0, contentsId, replacementId, { scope: "global" });
writer.end();

Modification uses incremental PDF updates, but this documentation does not make signature-preservation guarantees.