Recently, I had accidentally deleted a project file from SVN that was still required. Searching on how to recover it I came across different methods (mainly using svn copy) but an example using svn merge is what worked for me. All you need is the revision number that deleted the file(s) that you want back.
The following is the command format you'll want to use:
$ svn merge -r [rev]:[rev-1] [file-path-or-current-dir]
As an example, lets say I wanted to retrieve a post I had deleted for my site and the revision number that deleted the file was 50. That would look something like this:
$ cd Projects/website/posts/ $ svn merge -r 50:49 .
If you made other changes that were part of that revision they will be retrieved but are not committed so you can modify them as you like. In my case I had deleted 2 other files so I just re-deleted them, kept the one I needed and committed my changes. The revision history for the file you retain is preserved.