After a file has been uploaded to the tmp folder, for example a pdf file, would php fileinfo mime check be enough to verify that the file is indeed a pdf file and is not infected?
How do you verify that a user uploaded file has no virus?
so that I could let users download it?
My scenario is this: A user uploads a pdf file, I then let another user read his pdf file.
A mime check does not guarantee anything, to be reasonably sure that a file has no virus, you will have to pass it through a virus scanner, like for example clam on linux.
php fileinfo mime check be enough to verify that the file is indeed a pdf file and is not infected
No. It will tell you if it looks like a PDF file, but do no virus checking.
How do you verify that a user uploaded file has no virus?
With a virus scanner. There are plenty of commercial and non-commercial packages out there.