PDF thumbnails sometimes mostly black

Some pdfs don’t seem to thumbnail correctly.

For instance here’s a file and the thumbnail created

https://transfer.sh/wmWL3/apr1-apr1.pdf

https://transfer.sh/Qsot/-thumb-wd-apr1-apr1.jpg

See how the thumbnail is mostly black even though the file is mostly white?

If I use image magick convert on the command line, I use the ‘flatten’ option to prevent that. e.g.
convert -flatten -thumbnail 46x60 APR1_Apr1.pdf _new_thumb.jpg

gives you this better thumbnail

https://transfer.sh/VxuNr/-new-thumb.jpg

I’m no image magick expert, but please consider adding ‘flatten’ whatever the heck it does to your thumbnailing code…

https://ww.wp.xz.cn/plugins/file-away/