mklink errors out with "The device does not support symbolic links"

EDIT

I just noticed it looks a lot like you're trying to create a symlink between two network shares. It doesn't surprise me at all that this doesn't work properly. Why would you want to do such a thing? I got the same error when I tested just now too using Windows 10 and and two Linux Samba servers. I'm betting this functionality is just not supported, but if it is, perhaps it's only supported by Windows servers. I can't test that easily.

What OS do your storage servers run?

Original answer

According to this site, you may need to make some configuration changes for that to work. The site has better formatting, but I've quoted below in case the site goes away.

http://sourcedaddy.com/windows-7/how-to-create-symbolic-links-to-shared-folders.html

The mklink command also allows you to create a symbolic link targeting a Universal Naming Convention (UNC) path. For example, if you run the following command, Windows will create a symbolic link file called Link.txt that opens the Target.txt file.

Mklink link.txt \\server\folder\target.txt
If you enable remote symbolic links (discussed later in this section), they can be used to store symbolic links on shared folders and automatically redirect multiple Windows network clients to a different file on the network.

By default, you can use symbolic links only on local volumes. If you attempt to access a symbolic link located on a shared folder (regardless of the location of the target) or copy a symbolic link to a shared folder, you will receive an error. You can change this behavior by configuring the following Group Policy setting:

Computer Configuration\Administrative Templates\System\NTFS File System\Selectively Allow The Evaluation Of A SymbolicLink

When you enable this policy setting, you can select from four settings:

Local Link To Local Target Enabled by default, this allows local symbolic links to targets on the local file system.
Local Link To Remote Target Enabled by default, this allows local symbolic links to targets on shared folders.
Remote Link To Remote Target Disabled by default, this allows remote symbolic links to remote targets on shared folders.
Remote Link To Local Target Disabled by default, this allows remote symbolic links to remote targets on shared folders.
Enabling remote links can introduce security vulnerabilities. For example, a malicious user can create a symbolic link on a shared folder that references an absolute path on the local computer. When a user attempts to access the symbolic link, he will actually be accessing a different file that might contain confidential information. In this way, a sophisticated attacker might be able to trick a user into compromising the confidentiality of a file on his local computer.

Also I noticed you have a missing backslash in your UNC path, it should be \\fileserver1\files\test.pdf