Free version is listed in WordPress plugins repository and you may view it from here >>
You may download the package there and then click your WordPress Dashboard -> Plugins -> Add New -> Upload Plugin to upload the package & install the plugin.
Or you may click your WordPress Dashboard -> Plugins -> Add New -> Upload Plugin and enter keywords "bsk pdf manager". With the search results you just need to click Install Now then WordPress will help you to complete the remain work.
However you installed the plugin you need to activate the plugin.