For large libraries, you will not get a preview of the files.
If the total size of all library files is greater than 500 MB, the files will not get downloaded into Outlook all at once. This is because downloading such a large library can be slow and taxing. Instead, only the names of the files will appear, and the contents of the files will not be previewed automatically for you. In this situation, click the Download this document button at the bottom of the e-mail message. Then Outlook will download and preview the file for you.
Tip This isn't shown above, but here's something good to know for large libraries: In Outlook, right-click a file you want to download, and then click Mark to download document(s). After you’ve done that for several files, you can click the Send/Receive button to begin the download of only those files.