- Goto https://www.mediawiki.org/wiki/File:Bug36380.pdf
- download from the File history the lastest version to your computer (just to see that this works)
- create a modified version on your computer
- upload the modified file using https://www.mediawiki.org/w/index.php?title=Special:Upload&wpDestFile=Bug36380.pdf&wpForReUpload=1
After the upload of your changed version you will see that the preview image has NOT changed, and will not show your new version.
After clearing the browser cache with Ctrl-F5, you will see the correct preview image.