If Safari is still downloading the file, wait for it to finish. You can’t open a file while it’s being downloaded.
If the download is paused, resume it. Show the downloads list by clicking the Show Downloads button
, then click the Resume button 
for the paused download.Sometimes a download is incomplete because the file was damaged. Try downloading it again.
Make sure you have the app needed to open the file. If you do, the file may have been corrupted during the download. Delete it and try downloading it again.
The downloaded file may be a compressed archive or disk image that contains the file you want. If the file is from a trusted site, double-click it to decompress it.
If the trouble began after you installed a Safari plug-in, you can stop or remove the plug-in.