Product Version:

Adobe InDesign 2025 (version 20.x and later)
Operating System:

macOS 15.4
Summary:

Exporting a PDF directly from a book (.indb) using book.exportFile() produces a PDF with low-resolution images, even though the source links are updated and the documents contain high-resolution images. This issue started occurring in version 20.x and was not present in earlier versions.
Steps to Reproduce:

Create a book (.indb) containing multiple documents with high-resolution images.

Update links in the individual documents.

Use book.exportFile(ExportFormat.PDF_TYPE, ...) to export the entire book as a PDF.

The output PDF contains low-resolution images, noticeable especially when printing or zooming in.

Expected Result:

The exported PDF should contain images at full resolution, matching the quality of PDFs exported from individual documents using doc.exportFile().
Actual Result:

PDF exported from the book contains low-resolution images, differing from PDFs exported from individual documents.
Workaround:

Currently, the issue can be avoided by first merging the documents from the book into a single INDD file, updating the links, and then exporting the PDF from that merged document.
Attached Script:

// Repro script for PDF export issue from Book in InDesign 20.x+

try {
// Select the book file
var indbFile = File.openDialog("Select a book file (.indb)", "*.indb");
if (!indbFile) throw "No file was selected.";

// Open the book
var book = app.open(indbFile);

if (book.bookContents.length === 0) throw "The book contains no documents.";

// Update links in each document of the book
for (var i = 0; i < book.bookContents.length; i++) {
    var content = book.bookContents[i];
    var doc = app.open(content.fullName, true);

    var links = doc.links;
    for (var j = 0; j < links.length; j++) {
        if (links[j].status === LinkStatus.LINK_OUT_OF_DATE || links[j].status === LinkStatus.LINK_MISSING) {
            links[j].update();
        }
    }
    doc.save();
    doc.close(SaveOptions.NO);
}

// Select PDF export preset (first available)
if (app.pdfExportPresets.length === 0) throw "No PDF presets found.";
var pdfPreset = app.pdfExportPresets[0];

// Choose folder to save PDF
var outputFolder = Folder.selectDialog("Select folder to save PDF");
if (!outputFolder) throw "No folder was selected.";

var outputFile = File(outputFolder + "/" + indbFile.name.replace(/\.indb$/, "") + "_export_from_book.pdf");
book.exportFile(ExportFormat.PDF_TYPE, outputFile, false, pdfPreset);

alert("Export from book completed: " + outputFile.fsName);

book.close(SaveOptions.NO);

} catch (e) {
alert("Error: " + e);
}

Additional Notes:

The problem started with InDesign version 20.x.

Exporting PDFs from individual INDD documents works correctly.

Previous versions of InDesign did not exhibit this issue.