Add pdf library explanation

pdf/css/remark.css

@@ -44,4 +44,18 @@ table thead th {
 .full-width-pre > pre {
 	margin-left: -4em;
 	margin-right: -4em;
+}
+
+blockquote {
+  background: #dee2e6;
+  margin-left: 0;
+  padding: 0.5em;
+}
+
+blockquote > *:first-child {
+  margin-top: 0;
+}
+
+blockquote > *:last-child {
+  margin-bottom: 0
 }
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