Uploading a single file is as simple as:
curl -T local-filename.pdf -u 'username:password' "https://nextcloud.mydomain.tld/remote.php/dav/files/username/remote_filename.pdf
Note that the path above needs to include the username used to authenticate with. For example, I'd use something like
/justyn/ is the root of my main directory in Nextcloud.
Bonus example: Uploading all of the PDF files in a directory
This is roughly what I used to upload all of my PDF files in my scanner incoming directory. I did not want to include files that end in
.tif.pdf because those are single pages, where as the file ending in just
find . -type f -name \*.pdf -a ! -name \*.tif.pdf | while read x;do filename=$(basename $x); echo $filename;curl -T $x -u 'scannerusername:password' "https://nextcloud.mydomain.tld/remote.php/dav/files/scannerusername/ScansnapInbox/$filename";done