A user came to me complaining that she is getting HTTP 404 Not Found in Internet Explorer when clicking a pdf document in a SharePoint Document Library to view it, as shown below:
Note: SharePoint won't allow you to upload any documents that have the following special characters as shown in the screen shot below:
After further investigation and testing, I came to the following conclusion. Although you can upload a document to a SharePoint Document Library that has "+" sign in its name (ex. filename+). However, when you try to view or open the document in a web browser we get the HTTP 404 error. I believe this is "By Design" and the workaround would be to rename the file by removing the "+" sign and users would be able to view the document in a web browser.