While it may be possible to use the PDF Embedder Free or PDF Embedder Premium version of the plugin on a paid account on a WordPress.com hosting site, it is problematic due to WordPress.com being a closed environment (i.e. you do not have access to security settings, .htaccess, or other server tools). For this reason, it is not possible to use the PDF Embedded Secure version of the plugin on a WordPress.com hosted site reliably, at least definitely not the Pro version with its Secur PDFs functionality.
It is recommended that any of the plugin versions only be used on a self-hosted WordPress(.org) website being hosted by someone other than WordPress.com.
No version of the plugin can be installed on a basic (free) WordPress.com site. To understand the difference between WordPress.com and self-hosted WordPress sites, see here.