• Copy locally and extract Jira bundled plugin jira-fileviewer-plugin-7.2.1.jar located at ./atlassian-jira/WEB-INF/atlassian-bundled-plugins
• In extracted folder open file-service.js and delete (or wrap in comment) following lines:

if (!featureManager.isFeatureEnabled("jira.fileviewer.disable.pdf")) {
    selectors.document.push("a[file-preview-type=document]");
}

addDocumentSelector: function addDocumentSelector(sel) {
    pushSingleOrArray(selectors.document, sel);
}

• Remove same lines from minified file-service-min.js.  You can use JS prettify and minify tools to do this.
• Save files. Put folder content in ZIP. Rename file to jira-fileviewer-plugin-7.2.1.jar, copy to server and restart Jira.