If you know the relative Url of the folder within SharePoint there is a straight forward way to access the files within that folder.
using Microsoft.SharePoint.Client; ... using (ClientContext clientContext = new ClientContext("https://something.sharepoint.com/sites/mysite/")) { Folder folder = clientContext.Web.GetFolderByServerRelativeUrl("/sites/mysite/root_folder/sub_folder"); // Load the Folder and Files into the ClientContext so we can access them clientContext.Load(folder); clientContext.Load(folder.Files); clientContext.ExecuteQuery(); foreach (var item in folder.Files) { // Load each file into the ClientContext to access the file information context.Load(item); context.ExecuteQuery(); } }