(Data lost?) PDFs as the attachments are always missing in Obsidian

I didn’t use file cleaner and the local files are still in the computer with the same file names.

For images as the attachments, they will be in notes forever and no issues at all.

But if I attached PDFs in the notes, few days after the PDFs will disappear and the notes show me “.pdf is not created, click to create”

What is the issue, and how can I resolve it?