Sharing a single or a bunch of files is introduced in Sync v2.5 and it's a Pro feature, available in Sync Home Pro and Sync Business, and on all mobile devices running Android. Free version of Sync can only receive shared files, but cannot initiate sharing.
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 3 days, and this can be changed in poweruser 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 (Android only).
By default on desktops files are stored into user's Downloads directory and this can be changed in Sync settings. On Android files go to Downloads/SyncDownloads directory on internal device's memory. See here for other peculiarities of this option on Android.
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 only if they have Pro license. 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).