As far as I know it is not possible to convert a folder containing AsciiDoc files to a pdf, a simple script could do it but the problem would be in what order do you want your files to be converted?
The simplest solution for you is to create your own content.adoc
file and use the include macro to select what files you want to convert and in what order, it could look something like this:
= Mastering the Lightning Network
include::01_introduction.asciidoc[]
include::02_getting_started.asciidoc[]
include::03_how_ln_works.asciidoc[]
include::04_node_client.asciidoc[]
include::05_node_operations.asciidoc[]
include::06_lightning_architecture.asciidoc[]
include::07_payment_channels.asciidoc[]
include::08_routing_htlcs.asciidoc[]
include::09_channel_operation.asciidoc[]
include::10_onion_routing.asciidoc[]
include::11_gossip_channel_graph.asciidoc[]
include::12_path_finding.asciidoc[]
include::13_wire_protocol.asciidoc[]
include::14_encrypted_transport.asciidoc[]
include::15_payment_requests.asciidoc[]
include::16_security_privacy_ln.asciidoc[]
include::17_conclusion.asciidoc[]
and you convert using asciidoctor-pdf content.adoc