synology diskstation – Synchronize some (but not all) directories with rsync and with a file extension filter, protecting one single folder?

From this directory structure:

/Customer/1 folder ONE/various subdirs and files
/Customer/2 folder TWO/various subdirs and files
/Customer/3 folder THREE/3.1 subdir1/various subdirs and files
/Customer/3 folder THREE/3.2 subdir2/various subdirs and files
..
/Customer/3 folder THREE/3.9 subdir9/various subdirs and files
/Customer/3 folder THREE/3.10 subdir10/various subdirs and files

I would like to synchronize all the Customer pdf files included in his subfolders in “/3 folder THREE” from 3.1 to 3.10, entirely excluding the folders “/1 folder ONE” and “/2 folder TWO” into a target destination folder:

/Customer/3 folder THREE/3.1 subdir1/only pdf files in their subfolders
/Customer/3 folder THREE/3.2 subdir2/only pdf files in their subfolders

I found a way to filter/synchronize only the files with the extension *.pdf with the rsync option --filter.

What I can’t find is a possibility to protect a specific target folder (“/3.10 subdir10”) from being deleted by the rsync option --delete and --prune-empty-dirs.

How can I instruct rsync to NOT delete EXTRA files in only one target folder /3.10 subdir10?

/Customer/3 folder THREE/3.10 subdir10/only pdf files from rsync source, but many others from other sources

(This target folder will be used by others and filled with additional files. Rsync should copy the files from source 3.10 to destination 3.10, but not delete files who doesn’t exist in his source).