There are unconvertable PDF files

Last night I found something really strange. There was a strange PDF file that I downloaded from the Net. It seems like just a regular Traditional Chinese document with nothing but a bunch of text in the content. But when I tried to copy and paste the text content in a NotePad, I got a bunch of garbage. And then I tried to convert it to HTML document, and I got a strange error saying something that I didn't understand. Notice that I was not using merely the reader, but the Adobe Acrobat 3D. So .... clueless.