Sharing a single or a bunch of files (as opposed to sharing a whole folder) is introduced in Sync v2.5 where it's a Pro feature, in Sync 2.6. it's available in Sync Free as well and on all mobile devices running Android and iOS.
Sharing a file is basically a data transfer operation. This transfer may contain one file, as well as a few files at a time.
To start sharing either use "+" -> Share file menu or drag&drop the wanted items to Sync. Multi select is applicable here. Sync rescans the files, hashes them and generates the link/QR code. The link prompts to set up expiration date - uncheck this parameter and link will never expire. By default it's valid for 3 days, and this can be changed in power user settings, option "share_file_ttl", max value allowed is 7. Everyone who gets the generated link can download the shared files, and there is no option to restrict the number of usage or ban some devices from using the link.
The receiving side shall paste the link though "+" -> "Enter a key or link" menu and pick the files' location. Or scan the QR code from mobile phone.
By default on desktops files are stored into user's Downloads directory and this can be changed in Sync settings.
If running Sync in configuration mode or on a NAS, changing files location can be done through config file, parameter "files_default_path". Here you can learn location of config file on NAS. When editing path on Windows, don't forget to comment out the backslash in path, so parameter looks like this
On Android files go to Downloads/SyncDownloads directory on internal device's memory. See here for other peculiarities of this option on Android, and here for peculiarities on iOS.
Once peers connect and download is finished, a green checkmark appears in status.
Files are clickable in UI and take you to a File manager.
1) This is a one-time one-way file transfer. It means that if the shared files change, this transfer gets invalidated and changes are not synced anymore. To sync the changed files, add them to Sync again and generate the new link.
Only unchanged files are synced.
2) Peers that receive the files, can share them further. They cannot change link expiration date.
3) If link has expired, you need to add the files to Sync again and generate the new link.
4) If destination peer already has files with the same name in selected directory, Sync will add (1) index to the new ones and show the corresponding message in UI.
4) Removing the file from Sync UI does not remove it from the device. Also, removing the file from device won't remove it from Sync UI, but the file won't be uploaded anymore.