Out of the box IIS has a limit of ~30MB for file uploads, which may not be enough for large PDF's (for example). This can be changed.
<system.webServer>
...
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="2147483648"/>
</requestFiltering>
</security>
...
</system.webServer>
Source: https://devnet.kentico.com/articles/file-upload-limitation-on-iis6-7