Regarding your mention of "solid paper," this usually refers to two distinct concepts depending on the context: 1. RAR File Context: "Solid" Archives

24 lb (90 gsm) paper, such as Hammermill , is often recommended for bookbinding because it is thicker than standard printer paper and less likely to "show-through" text.

A "solid" RAR archive treats all files as one continuous data stream. This allows for significantly higher compression ratios, especially when the archive contains many similar small files.