Web browsers have a "cache", which is a memory allowance. This can fill up pretty quickly when downloading big files. Another potential problem is that perhaps the file downloaded was created in a version higher than your PDF reader.
Solution 1: Empty your web browser's "cache", that is, empty out its internet history.
Explorer: Choose "Tools", then "Internet Options" and then click on the "Delete" button under "Browsing history". Firefox: Choose "Tools" and then "Delete Recent history". Google Chrome: Choose the tools menu, then "Options" and "Under the Hood", and click on the "Clear recent data button". Choose the "Clear download history", "Clear browsing history" and "Empty cache". Safari: Choose the general Safari settings menu, then "Reset Safari" and then "Empty the cache" and Clear history".
Solution 2: Go to http://get.adobe.com/reader/ to get and update your Adobe Acrobat Reader (it is free) to the latest version.