Under Media, select one of these PDFs and click the i icon. What is its mime type? Make sure it is listed in Legal MIME Types under the Media settings.

This is better posted @ https://forum.joomla.org/ as it appears to be a support question specific to your setup.

The file is listed as Mime Type: "application/pdf" and extension: "pdf"

pdf is configured as a legal document extension, and application/pdf is configured as a legal mime type.

I am not able to reproduce it with v4.3.4. Maybe you can test with a separate clean installation using the latest version.

1. When they are unable to upload the pdf an error message is displayed. What is it?
2. Long shot but is fileinfo enabled you can find out at administrator/index.php?option=com_admin&view=sysinfo)

Can you please share your media manager options? I suspect that you've followed some wrong tutorial or you have some plugin that is really bad (assigns pdf to images)...

@brianteeman The error states that the file uploaded is not a valid image, Fileinfo Available displays Yes.

@dgrammatiko
Maximum Size (MB): 15
Path to files: images
Path to images: images
Restrict uploads: Yes
Allowed extensions: bmp,gif,jpg,jpeg,png,webp,ico,mp3,mp4,odg,odp,ods,odt,pdf,png,ppt,txt,xcf,xls,csv,doc,docx,xlsx,pptx,dotx,zip
Check MIME-types: Yes
Allowed image extensions: bmp,gif,jpg,png
Legal Audio Extensions: mp3,m4a,mp4a,ogg
Legal Video Extensions: mp4,mp4v,mpeg,mov,webm
Legal Document Extensions: doc,odg,odp,ods,odt,pdf,ppt,txt,xcf,xls,csv,docx,xlsx,pptx,zip,dotx
Illegal extensions: blank
Allowed MIME-types:

image/jpeg,image/gif,image/png,image/bmp,application/x-shockwave-flash,
application/msword,application/excel,application/pdf,application/powerpoint,
text/plain,application/x-zip,application/vnd.openxmlformats-officedocument.wordprocessingml.document,
application/vnd.openxmlformats-officedocument.wordprocessingml.template,application/vnd.ms-word.document.macroEnabled.12,
application/vnd.ms-word.template.macroEnabled.12,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet,
application/vnd.openxmlformats-officedocument.spreadsheetml.template,application/vnd.openxmlformats-officedocument.presentationml.presentation,
application/vnd.openxmlformats-officedocument.presentationml.slideshow,video/mp4,
application/zip,application/x-7z-compressed,application/zip-compressed,application/x-zip-compressed,
application/octet-stream,multipart/x-zip,application/x-compress,application/x-compressed

Forbidden MIME-types: text/html

I am not sure if this is relevant here. I cannot upload pdfs. I have a very similar problem with [https://forum.joomla.org/viewtopic.php?t=1002182]
However i found an error in the console.
/administrator/index.php?option=com_media&format=json&mediatypes=0,1,2,3&task=api.files&path=local-images:/content/pdfs/2023](administrator/index.php?option=com_media&format=json&mediatypes=0,1,2,3&task=api.files&path=local-images%3A%2Fcontent%2Fpdfs%2F2023
[HTTP/1.1 403 Forbidden 1332ms]
the http headers plugin is disabled.
If you think is relevant here i can add more info.
Tje strange thing is that when i worked this site in a development server file uploading worked !

Closing as having a pull request. See #43345 .