(Maybe this is better for the development form, but I'm asking to try to figure out how things work, with no intent to do development work.)
What does the following notation mean in the center page of a note on Windows 10? (Second line does not show up in Android.)
[c869dace2120430a8038b34d19ef725a.pdf](:/950a7db5c7d64a03870dddc9a665abdb) is a link, apparently to a PDF.
Links internal to Joplin take the format
[display_text](:/joplin_unique_id) - no space or newline between
c869dace2120430a8038b34d19ef725a.pdf is the file's name (it's a random string so the file was possibly added by pasting from the clipboard).
:/950a7db5c7d64a03870dddc9a665abdb is Joplin's unique ID for the file itself as stored in its
External links are similarly formatted.
[link text displayed in the note](URL)
[Link to the Joplin Forum](https://discourse.joplinapp.org)
See also the Joplin Markdown Guide.