It should be that you only downloaded the link of the file when you downloaded it, but didn't download the complete file to the computer. Generally speaking, if it is a journal paper, you can directly click the download tab on the left side of the article. If it is a dissertation, you need to click to open the dissertation you searched, and then choose to download the full text. See if the files you downloaded are very small, with a capacity of only tens of K. If so, that should be the reason I said. You can download the full text again.