Background Task Toggle on/off
The Background Task hangs when making a PDF. Add a toggle on/off of the Background Tasks to force a foreground PDF export. This has been an issue for many years and still isn't corrected!

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monokano commented
In rare cases, PDF files exported from InDesign have defects such as missing images (alerts are not displayed). The cause is thought to be PDF output in the background. Therefore, users add “DisableAsyncExports.txt” to avoid PDF file failure. This is a very cumbersome operation. Users have been waiting for years to be easily turned off in the background task panel.
Japanese:
InDesignから書き出されたPDFファイルは、まれに画像が欠ける等の不良が発生します(アラートは表示されません)。その原因はバックグラウンドでのPDF書き出しであると考えられています。そのためユーザーはPDFファイル不良発生を回避するために「DisableAsyncExports.txt」を追加します。これはとても面倒な操作です。ユーザーはバックグラウンドタスクパネルで容易にOFFにできることを長年の間待ち続けています。 -
rupfi commented
The https://www.publishingblog.ch writes about it:
There is a very simple way in InDesign to bring these file exports back to the foreground. Create an empty text file - with a text editor and not with a word processor! - and calls it "DisableAsyncExports.txt". Copy this file into the InDesign program directory.
But I haven't tried it out to see if it works.