Retagging keeps the xref stream, so Preview opens the output

src/tag.ts

@@ -32,7 +32,9 @@ export function tag(pdf: Uint8Array, input: PagesInput, opts: TagOptions = {}, r
   const { doc } = src;
   // The old overlay's text is gone on purpose, so the text check compares against the source without it.
   // Pixels compare against the input itself: the restore must not change a pixel.
-  const baseline = src.restored ? save(doc, true) : pdf;
+  // The baseline is saved from a second copy: after a full save, mupdf writes the
+  // incremental update as an xref table with the xref stream's trailer, which Preview cannot open.
+  const baseline = src.restored ? save(openPdf(pdf, opts).doc, true) : pdf;
   const warn = (w: Warning) => report.warnings.push(w);
   src.warnings.forEach(warn);
   const pageCount = doc.countPages();

test/document.test.ts

@@ -84,6 +84,22 @@ test("retags a PDF that is already tagged", () => {
   assert.equal(report.verification.differingPixels, 0);
 });
 
+test("retagging a PDF with an xref stream appends an xref stream", () => {
+  // A classic table there, carrying the stream's trailer, is a file macOS Preview cannot open.
+  const src = new mupdf.PDFDocument(readFixture("text-simple.pdf")).saveToBuffer("compress,objstms").asUint8Array().slice();
+  const lastXref = (pdf: Uint8Array) => {
+    const s = Buffer.from(pdf).toString("latin1");
+    const at = Number(s.slice(s.lastIndexOf("startxref") + 9).trim().split(/\s/)[0]);
+    return s.startsWith("xref", at) ? "table" : "stream";
+  };
+  assert.equal(lastXref(src), "stream");
+  const once = tag(src, simple);
+  const report = newReport();
+  const twice = tag(once, simple, {}, report);
+  assert.ok(report.warnings.some((w) => w.code === "retagged"));
+  assert.deepEqual([lastXref(once), lastXref(twice)], ["stream", "stream"]);
+});
+
 test("retagging drops a foreign tag tree and what points into it", () => {
   const pdf = blankPdf(1, (doc) => {
     const page = doc.findPage(0);