Have you checked these first?
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Help us help you: include your environment details. We've made this easy for you. Use the Copy System Info button in Admin Panel › osAwesome › Diagnostics, then paste below.
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Hello
I have the attachment display inline plugin, working normally. I also have the attachment storage plugin on Amazon S3.
The system normally stores attachments in S3, i can download it from osticket, but is unable to display these attachments inline in the ticket thread.
If i change the storage to filesystem, works normally. But because of our storage size, i need to use S3 buckets.

osTicket v1.16.3 // osTicket-1.16.3-Awesome-101
PHP 8.0.25 // MySQL 10.3.34 // Apache web server
Just to add an information, attachments in JPG format works normally.
What is not working is with the PDF files.
When you're looking at one of these tickets, what happens if you press SHIFT + O on your keyboard? Can you see the inline attachment?