Using your default browser is the main reason why you can't open pdf files. I have the same problem after downloading a pdf file using original browser with my asus smart phone.
I got this error when trying to open the pdf file using adobe reader.
The document can't be opened because it is not a valid pdf document.
After a few research i found out that we just need to download the pdf file using different browser. For my case I use Google chrome.
It seems that some browser does not download certain file properly or have restrictions for security purpose. And there are cases where chrome does not work but opera did. The reason for this problem could be the android system or the android browser apps. You can also try to request desktop site instead of mobile site and avoid to click download button before the page finish loading.