1. Use filepath.Glob if you want to glob

Most of [0] can be replaced with filepath.Glob. You should probably also respect TMPDIR if it's set, which ioutil.TempDir etc will do.

2. ExitErr shouldn't be used, especially not within nested functions.

You have 'defer os.RemoveAll(tmpDir)'. defers won't get called if you `os.Exit(1)` as you do in ExitErr. You should be instead doing `return nil, err` all the way back up to `err := RunApp()`, and printing the error there. That will let defers run. In general, using os.Exit anywhere other than in the main function is an antipattern.

3. Your parallel processing should be using a waitgroup or errgroup. A slice of channels is super fragile.

In [1], do something like the following:

    // before the for loop
    var wg sync.WaitGroup
   
    // in each iteration of the for loop
    wg.Add(1)
    go func(name string) {
      defer wg.Done()
      cli.ImageRoutine(fileName)
    }(fileName)

    // at the end, to wait
    wg.Wait()

The channel thing you're doing now is more fragile than a waitgroup, and harder to reason about.

4. use of exported functions / unexported functions is all over the place and inconsistent.

There's a few other things, but that's what I've got offhand. I can't actually run the code because I only use linux, and it obv doesn't run on linux. Oh, I guess that's another thing, you want some build tags to make it more clear it won't run on linux.

Hopefully something in there was helpful

[0]: https://github.com/rootVIII/pdfinverter/blob/5fe9f505779bb9d...

[1]: https://github.com/rootVIII/pdfinverter/blob/6cbcd4cc7254514...