Hi @liubing2007,
Thanks for reaching out, and I’m so sorry about the delay in getting back to you.
Please confirm first that you are using the latest version of the free plugin (version 4.8.2).
Once confirmed that you are using the latest version, please know that these types of issues can be caused by caching/minifying plugins where they create issues with the plugin’s functions, such as the PDFs failing to load properly or throwing errors.
To rectify this, you will need to exclude the URIs for the plugin scripts from any caching and/or minifying tools.
Please see our guide here on the exact URIs to exclude: https://wp-pdf.com/kb/using-caching-or-minifying-plugins/.
The plugin scripts are already minified, so you will not be losing any efficiency or load speed by excluding them.
Be sure to flush any caches after you exclude the scripts so that they can be loaded by the site again.
I hope this helps. Thanks ??