When 'creating thumbnails during indexing' is not disabled in the Settings, thumbnails will be created for all new documents. This can take a while and there might even be documents causing the pdf-plug-in to crash or hang so the indexing will not finish at all. Unfortunately indexing does not give much feedback on its progress.
If thumbnails are not generated during indexing, they are created when opening a documents for the first time. So eventually all documents will get a thumbnail.
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