Can I open a PDF file that is possibly locked on mac? I have one book that I purchased online, and the company seems to allow users to read the book only through their app (which I'm not 100% sure, but the error message below implies it).
However, I could no longer access the account so I could not read it on app. The book data is under ~/Library/ApplicationSupport as PDF file. There is another file with an extension .dat on the same directory.
But when I tried opening the PDF file, the dialog appeared, saying it may be damaged or use a file format that preview doesn't recognize. I tried installing Adobe Acrobat Reader but there appears the same error message.
Is there any way to read the PDF file, possibly on iBooks?
Some vendors of PDFs do encrypt them in third-party DRM, which requires the use of a special reader, or a plug-in for Adobe Reader/Acrobat. (Such as FileOpen).
Realistically, you need to ask the vendor about whether the files they supply can be read on standard PDF readers, and what to do about recovering access to your account.
It's arguable whether such files are 'true' PDFs. The first four characters of a 'normal' PDF file should be %PDF, so you can open a PDF file in a text editor and see if the files you've found are indeed PDFs. (Of course, it's possible for them to be incomplete or damaged even if they have this header.)
I'm not sure whether iBooks can handle DRM: It's likely just the same PDF engine as is used in Preview.