Because this runs as a Gradle worker the actual classpath is
..../caches/..../asciidoctorj-pdf-1.5.0-alpha.16.jar
..../caches/..../asciidoctorj-1.6.0-RC.2.jar
..../caches/..../asciidoctorj-diagram-1.5.4.1.jar
..../caches/..../jruby-complete-9.2.4.0.jar
..../caches/..../asciidoctorj-api-1.6.0-RC.2.jar
..../caches/..../jcommander-1.35.jar
..../asciidoctor-gradle-jvm/build/classes/java/gradleTest
..../asciidoctor-gradle-jvm/build/classes/groovy/gradleTest
..../asciidoctor-gradle-jvm/build/resources/gradleTest
..../lib/groovy-all-2.4.15.jar
..../.gradle/caches/4.10.2/generated-gradle-jars/gradle-test-kit-4.10.2.jar
..../.gradle/caches/4.10.2/generated-gradle-jars/gradle-api-4.10.2.jar
..../lib/gradle-installation-beacon-4.10.2.jar
..../caches/....org.spockframework/spock-core/1.1-groovy-2.4/1c42014d4a18995d5d4aa10a4e060e88f60fdf78/spock-core-1.1-groovy-2.4.jar
..../caches/....junit/junit/4.12/2973d150c0dc1fefe998f834810d68f278ea58ec/junit-4.12.jar
..../caches/....org.hamcrest/hamcrest-core/1.3/42a25dc3219429f0e5d060061f71acb49bf010a0/hamcrest-core-1.3.jar
..../asciidoctor-gradle-jvm/build/pluginUnderTestMetadata
..../asciidoctor-gradle-base/build/libs/asciidoctor-gradle-base-2.0-alpha.4.jar
..../caches/....org.codehaus.groovy/groovy-all/2.4.15/423a17aeb2f64bc6f76e8e44265a548bec80fd42/groovy-all-2.4.15.jar
..../testfixtures/jvm/build/libs/testfixtures-jvm-2.0-alpha.4.jar
..../caches/....org.ysb33r.gradle/grolifant/0.10/e2ad2b34631cd8f19f2a992984a0dc4c71bee78f/grolifant-0.10.jar
..../caches/....org.tukaani/xz/1.6/5b6f921f1810bdf90e25471968f741f87168b64/xz-1.6.jar
..../caches/....commons-io/commons-io/2.6/815893df5f31da2ece4040fe0a12fd44b577afaf/commons-io-2.6.jar
..../caches/....org.jsoup/jsoup/1.11.2/e3eeb8a0b4ce1db246059a41e353cd7413dad226/jsoup-1.11.2.jar
..../caches/....cglib/cglib-nodep/3.2.6/92bf48723d277d6efd1150b2f7e9e1e92cb56caf/cglib-nodep-3.2.6.jar
..../asciidoctor-gradle-jvm/build/classes/java/main
..../asciidoctor-gradle-jvm/build/classes/groovy/main
..../asciidoctor-gradle-jvm/build/resources/main
..../lib/plugins/gradle-workers-4.10.2.jar
..../lib/jcip-annotations-1.0.jar
..../lib/plugins/gradle-dependency-management-4.10.2.jar
..../lib/commons-lang-2.6.jar
..../lib/asm-6.2.jar
..../lib/asm-commons-6.2.jar
..../lib/asm-tree-6.2.jar
..../lib/asm-analysis-6.2.jar
..../lib/plugins/asm-util-6.2.jar
..../lib/plugins/bcprov-jdk15on-1.58.jar
..../lib/commons-io-2.4.jar
..../lib/plugins/gson-2.7.jar
..../lib/plugins/ivy-2.2.0.jar
..../lib/plugins/maven-core-3.0.4.jar
..../lib/plugins/aether-api-1.13.1.jar
..../lib/plugins/aether-util-1.13.1.jar
..../lib/plugins/aether-impl-1.13.1.jar
..../lib/plugins/aether-spi-1.13.1.jar
..../lib/plugins/maven-artifact-3.0.4.jar
..../lib/plugins/plexus-utils-3.0.8.jar
..../lib/plugins/maven-settings-builder-3.0.4.jar
..../lib/plugins/plexus-component-annotations-1.5.5.jar
..../lib/plugins/plexus-interpolation-1.14.jar
..../lib/plugins/plexus-sec-dispatcher-1.3.jar
..../lib/plugins/plexus-cipher-1.7.jar
..../lib/plugins/maven-settings-3.0.4.jar
..../lib/plugins/maven-model-3.0.4.jar
..../lib/plugins/maven-model-builder-3.0.4.jar
..../lib/plugins/maven-aether-provider-3.0.4.jar
..../lib/plugins/maven-repository-metadata-3.0.4.jar
..../lib/slf4j-api-1.7.16.jar
..../lib/plugins/gradle-composite-builds-4.10.2.jar
..../lib/gradle-launcher-4.10.2.jar
..../lib/gradle-tooling-api-4.10.2.jar
..../lib/gradle-wrapper-4.10.2.jar
..../lib/plugins/gradle-version-control-4.10.2.jar
..../lib/plugins/httpclient-4.5.5.jar
..../lib/plugins/commons-codec-1.10.jar
..../lib/plugins/httpcore-4.4.9.jar
..../lib/jcl-over-slf4j-1.7.16.jar
..../lib/plugins/org.eclipse.jgit-4.5.3.201708160445-r.jar
..../lib/plugins/jsch-0.1.54.jar
..../lib/plugins/gradle-resources-http-4.10.2.jar
..../lib/guava-jdk5-17.0.jar
..../lib/plugins/jcifs-1.3.17.jar
..../lib/plugins/nekohtml-1.9.20.jar
..../lib/plugins/xercesImpl-2.12.0.jar
..../lib/xml-apis-1.4.01.jar
..../lib/gradle-runtime-api-info-4.10.2.jar
..../lib/plugins/gradle-plugin-use-4.10.2.jar
..../lib/gradle-kotlin-dsl-provider-plugins-1.0-rc-6.jar
..../lib/annotations-13.0.jar
..../lib/gradle-kotlin-dsl-1.0-rc-6.jar
..../lib/gradle-kotlin-dsl-tooling-models-1.0-rc-6.jar
..../lib/kotlin-compiler-embeddable-1.2.61.jar
..../lib/kotlin-reflect-1.2.61.jar
..../lib/kotlin-sam-with-receiver-compiler-plugin-1.2.61.jar
..../lib/kotlin-script-runtime-1.2.61.jar
..../lib/kotlin-stdlib-1.2.61.jar
..../lib/kotlin-stdlib-common-1.2.61.jar
..../lib/kotlin-stdlib-jdk7-1.2.61.jar
..../lib/kotlin-stdlib-jdk8-1.2.61.jar
..../lib/gradle-kotlin-dsl-tooling-builders-1.0-rc-6.jar
..../lib/plugins/gradle-announce-4.10.2.jar
..../lib/plugins/gradle-antlr-4.10.2.jar
..../lib/plugins/gradle-build-cache-http-4.10.2.jar
..../lib/plugins/gradle-build-comparison-4.10.2.jar
..../lib/plugins/gradle-build-init-4.10.2.jar
..../lib/plugins/plexus-container-default-1.5.5.jar
..../lib/plugins/plexus-classworlds-2.4.jar
..../lib/plugins/xbean-reflect-3.4.jar
..../lib/log4j-over-slf4j-1.7.16.jar
..../lib/plugins/maven-compat-3.0.4.jar
..../lib/plugins/wagon-provider-api-2.4.jar
..../lib/plugins/maven-plugin-api-3.0.4.jar
..../lib/plugins/gradle-code-quality-4.10.2.jar
..../lib/plugins/gradle-diagnostics-4.10.2.jar
..../lib/commons-collections-3.2.2.jar
..../lib/plugins/jatl-0.2.2.jar
..../lib/plugins/gradle-ear-4.10.2.jar
..../lib/javax.inject-1.jar
..../lib/plugins/gradle-ide-4.10.2.jar
..../lib/plugins/gradle-ide-native-4.10.2.jar
..../lib/plugins/dd-plist-1.20.jar
..../lib/plugins/gradle-ide-play-4.10.2.jar
..../lib/plugins/gradle-ivy-4.10.2.jar
..../lib/plugins/gradle-jacoco-4.10.2.jar
..../lib/plugins/gradle-javascript-4.10.2.jar
..../lib/plugins/rhino-1.7R3.jar
..../lib/plugins/simple-4.1.21.jar
..../lib/plugins/gradle-language-groovy-4.10.2.jar
..../lib/plugins/gradle-language-java-4.10.2.jar
..../lib/plugins/gradle-language-jvm-4.10.2.jar
..../lib/plugins/gradle-language-native-4.10.2.jar
..../lib/plugins/gradle-language-scala-4.10.2.jar
..../lib/plugins/gradle-maven-4.10.2.jar
..../lib/plugins/aether-connector-wagon-1.13.1.jar
..../lib/plugins/wagon-file-2.4.jar
..../lib/plugins/wagon-http-2.4.jar
..../lib/plugins/wagon-http-shared4-2.4.jar
..../lib/plugins/pmaven-common-0.8-20100325.jar
..../lib/plugins/pmaven-groovy-0.8-20100325.jar
..../lib/plugins/gradle-osgi-4.10.2.jar
..../lib/plugins/biz.aQute.bndlib-3.4.0.jar
..../lib/plugins/gradle-platform-base-4.10.2.jar
..../lib/plugins/gradle-platform-jvm-4.10.2.jar
..../lib/plugins/gradle-platform-native-4.10.2.jar
..../lib/plugins/snakeyaml-1.6.jar
..../lib/plugins/gradle-platform-play-4.10.2.jar
..../lib/plugins/gradle-plugin-development-4.10.2.jar
..../lib/plugins/gradle-plugins-4.10.2.jar
..../lib/ant-1.9.11.jar
..../lib/ant-launcher-1.9.11.jar
..../lib/plugins/commons-cli-1.2.jar
..../lib/plugins/gradle-publish-4.10.2.jar
..../lib/plugins/gradle-reporting-4.10.2.jar
..../lib/plugins/gradle-resources-gcs-4.10.2.jar
..../lib/plugins/google-api-services-storage-v1-rev116-1.23.0.jar
..../lib/plugins/google-api-client-1.23.0.jar
..../lib/plugins/google-oauth-client-1.23.0.jar
..../lib/jsr305-2.0.1.jar
..../lib/plugins/google-http-client-1.23.0.jar
..../lib/plugins/google-http-client-jackson2-1.23.0.jar
..../lib/plugins/jackson-core-2.9.4.jar
..../lib/plugins/jackson-annotations-2.9.4.jar
..../lib/plugins/jackson-databind-2.9.4.jar
..../lib/plugins/joda-time-2.8.2.jar
..../lib/plugins/gradle-resources-s3-4.10.2.jar
..../lib/plugins/aws-java-sdk-core-1.11.267.jar
..../lib/plugins/ion-java-1.0.2.jar
..../lib/plugins/aws-java-sdk-s3-1.11.267.jar
..../lib/plugins/jmespath-java-1.11.267.jar
..../lib/plugins/aws-java-sdk-kms-1.11.267.jar
..../lib/plugins/gradle-resources-sftp-4.10.2.jar
..../lib/plugins/gradle-scala-4.10.2.jar
..../lib/plugins/gradle-signing-4.10.2.jar
..../lib/plugins/bcpg-jdk15on-1.58.jar
..../lib/plugins/gradle-testing-base-4.10.2.jar
..../lib/kryo-2.20.jar
..../lib/objenesis-1.2.jar
..../lib/minlog-1.2.jar
..../lib/reflectasm-1.07-shaded.jar
..../lib/plugins/gradle-testing-junit-platform-4.10.2.jar
..../lib/plugins/junit-platform-launcher-1.0.3.jar
..../lib/plugins/apiguardian-api-1.0.0.jar
..../lib/plugins/junit-platform-engine-1.0.3.jar
..../lib/plugins/opentest4j-1.0.0.jar
..../lib/plugins/junit-platform-commons-1.0.3.jar
..../lib/plugins/gradle-testing-jvm-4.10.2.jar
..../lib/plugins/bsh-2.0b6.jar
..../lib/plugins/testng-6.3.1.jar
..../lib/plugins/jcommander-1.47.jar
..../lib/plugins/gradle-testing-native-4.10.2.jar
..../lib/plugins/gradle-tooling-native-4.10.2.jar
..../lib/plugins/gradle-tooling-api-builders-4.10.2.jar
..../lib/commons-compress-1.14.jar
..../lib/fastutil-7.2.1-min.jar
..../lib/gradle-api-metadata-4.10.2.jar
..../lib/gradle-base-services-4.10.2.jar
..../lib/gradle-base-services-groovy-4.10.2.jar
..../lib/gradle-build-cache-4.10.2.jar
..../lib/gradle-build-option-4.10.2.jar
..../lib/gradle-cli-4.10.2.jar
..../lib/gradle-core-4.10.2.jar
..../lib/gradle-core-api-4.10.2.jar
..../lib/gradle-docs-4.10.2.jar
..../lib/gradle-jvm-services-4.10.2.jar
..../lib/gradle-logging-4.10.2.jar
..../lib/gradle-messaging-4.10.2.jar
..../lib/gradle-model-core-4.10.2.jar
..../lib/gradle-model-groovy-4.10.2.jar
..../lib/gradle-native-4.10.2.jar
..../lib/gradle-persistent-cache-4.10.2.jar
..../lib/gradle-process-services-4.10.2.jar
..../lib/gradle-resources-4.10.2.jar
..../lib/jansi-1.14.jar
..../lib/jul-to-slf4j-1.7.16.jar
..../lib/native-platform-0.14.jar
..../lib/native-platform-freebsd-amd64-libcpp-0.14.jar
..../lib/native-platform-freebsd-amd64-libstdcpp-0.14.jar
..../lib/native-platform-freebsd-i386-libcpp-0.14.jar
..../lib/native-platform-freebsd-i386-libstdcpp-0.14.jar
..../lib/native-platform-linux-amd64-0.14.jar
..../lib/native-platform-linux-amd64-ncurses5-0.14.jar
..../lib/native-platform-linux-amd64-ncurses6-0.14.jar
..../lib/native-platform-linux-i386-0.14.jar
..../lib/native-platform-linux-i386-ncurses5-0.14.jar
..../lib/native-platform-linux-i386-ncurses6-0.14.jar
..../lib/native-platform-osx-amd64-0.14.jar
..../lib/native-platform-osx-i386-0.14.jar
..../lib/native-platform-windows-amd64-0.14.jar
..../lib/native-platform-windows-i386-0.14.jar
..../.gradle/caches/4.10.2/workerMain/gradle-worker.jar
..../asciidoctor-gradle-jvm/build/tmp/expandedArchives/org.jacoco.agent-0.8.1.jar_8059ed6e1ab8b88aac5d9097fad847bb/jacocoagent.jar
Look at it carefully and notice the snakeyaml 1.6 that is leaked by Gradle onto the worker classpath. Basically that snakeyanl will appear on the classpath before the one unpacked by jruby-complete.
Currently the only solution is to run the PDF backend using normal javaexec
and not a worker daemon.