Avoid 1.41.7.x to 1.42.0.x due to security issues and associated remote access restrictions. See Plex Media Server - Security Update - #3 by PlexInfo. Also, some versions of 1.41.7 are afflicted with the “database bloat” bug, so best avoided altogether.
Do not fall back below 1.40 due to database changes (and IIRC, 1.32.8 had DVR issues).
Look at the URLs below. The first is for the DSM 7 version. The second is for the DSM 7.2.2+ version.
Notice the two extra "2"s in the DSM 7.2.2+ version?
Find the version you want and adjust accordingly.
https://downloads.plex.tv/plex-media-server-new/1.42.2.10156-f737b826c/synology-dsm7/PlexMediaServer-1.42.2.10156-f737b826c-x86_64_DSM7.spk
https://downloads.plex.tv/plex-media-server-new/1.42.2.10156-f737b826c/synology-dsm72/PlexMediaServer-1.42.2.10156-f737b826c-x86_64_DSM72.spk
