This rejection is caused by a non- PDF document that has been saved or renamed with a .pdf file extension.

you don't see the %pdf-header because that's part of the file's metadata that's contained within the file.

most versions of turbotax don't allow the e-filing of any attachments.  rather they must be submitted separately with form 8453.  only certain forms are supported by the 8453. if your's is not one of them the entire return will have to be mailed in with the attachments printed out. 

Do you know how I can see which file is causing the error?

[removed, by me]

SOLVED: 

After scouring the forums, I found a post that said the cause of this error is most likely from the 1099B upload. TurboTax asks you to upload a copy of that document to verify the info you entered. Here's what I did exactly: 

1. Found the 1099B section (in my Federal return)

2. Pressed edit, and then went to the section where it showed my uploaded document

3. Delete that upload

4. Open the document you sent on your computer in Acrobat Reader

5. RESAVE it as a PDF

6. Upload the NEW pdf to your 1099B section. 

7. Save that edit. And re-file. 

That was it. I honestly don't know why it didn't like the first pdf I submitted, I'm not a coder or anything. But this worked. My e-file was very quickly approved after submitting this time.