Yilin Zhou 

Have you installed the IRIS plugin?

Does the out of the box PDF filetype extract any text at all?

Is it possible to see the PDF?

I have installed IRIS plugin

And here's my setting for IRIS

I also tried to untick "use IRIS technology for OCR" and ticked it again, but then it showed this

And my file is still not accessible

Yilin Zhou 

So can we see the file?  PDFs can be tricky and often the best approach is to handle them outside of Studio to convert into something that is actually translatable.  

Yilin Zhou 

I think this is an example of a PDF that should be handled outside of Trados Studio to create a translatable file you can work with.  I had problems using Acrobat to edit this file, and in Word I get this message:

Given the Studio PDF filetype uses Word as it converts the file to DOCX first this may be a related problem.

I think your best bet for a file like this is to use something like Abbyy Finereader, or even a tool like https://pdfsimpli.com/ which will allow you to edit the source with text you can use in a translatable file.

Unfortunately PDF files can require a lot of work to translate and it is always best to try and get the original source file for translation.