usuario servicio apache y carpetas compartidas en windows

[emprear]

Configuring Access to Network Resources

Access to files over the network can be specified using two mechanisms provided by Windows:
Mapped drive letterse.g., Alias /images/ Z:/UNC pathse.g., Alias /images/ //imagehost/www/images/
Mapped drive letters allow the administrator to maintain the mapping to a specific machine and path outside of the Apache httpd configuration. However, these mappings are associated only with interactive sessions and are not directly available to Apache httpd when it is started as a service. Use only UNC paths for network resources in httpd.conf so that the resources can be accessed consistently regardless of how Apache httpd is started. (Arcane and error prone procedures may work around the restriction on mapped drive letters, but this is not recommended.)

Example DocumentRoot with UNC path
DocumentRoot //dochost/www/html/
Example DocumentRoot with IP address in UNC path
DocumentRoot //192.168.1.50/docs/
Example Alias and corresponding Directory with UNC path
Alias /images/ //imagehost/www/images/
<Directory //imagehost/www/images/>
...
<Directory>
When running Apache httpd as a service, you must create a separate account in order to access network resources, as described above.

Es decir, usá solo la ruta al recurso compartido

DocumentRoot //192.168.1.50/docs/
Alias /pdf/ //192.168.1.50/archivos_pdf/

Espero sea la solución, al menos en Win7 y corriendo como administrador, funciona

Saludos