The iCloud files on the iPad are "not downloaded" after being unused for a while

I have an iPad Pro under iOS 12.2. I manage my files via the available file manager (mainly PDFs). After a period of non-use, local copies are deleted and a cloud sign appears. The redownload can not be done at the same time and each file must be redownloaded separately. I need my conference slides to stay as a local copy. There is no parameter to optimize storage that would do this to files (only for applications and music).