File Downloading Error: Open as aspx

Hi, I have one file of size 270MB of type PDF, which I had attached to one document under the Files tab. but when I try to download the same file, the file is open in named as vaultserver.aspx all file content I can show on the browser but I am not able to save it as PDF which is its original format. Other files that have a size of 2 to 3 MB get downloaded in PDF format. Is there file size issue? or something else? Regards, Maddy.