Merge pdf files. Merged files are not released and cannot be deleted

[lako12]

I have a Java method that needs to read the PDF files inside a folder and create a file corresponding to the merge of all those inside the folder. Finally I have to delete the files and leave only the result in the folder. The problem is that once the final file is created, I can't delete all the other files used for the merge, as if it doesn't release them after use. I also tried to delete them manually from the file explorer, but I can't (as long as the application remains running). I'm using this version:(Java 11)

        <dependency>
            <groupId>com.github.librepdf</groupId>
            <artifactId>openpdf</artifactId>
            <version>1.4.1</version>
        </dependency>

My Code:

        File[] files = directory.listFiles();
        if (files != null && files.length > 0) {
            try (Document document = new Document()) {
                PdfCopy copy = new PdfCopy(document, new FileOutputStream(fileName));
                document.open();

                for (File file : files) {
                    try (PdfReader reader = new PdfReader(file.getAbsolutePath())) {
                        for (int i = 1; i <= reader.getNumberOfPages(); i++) {
                            copy.addPage(copy.getImportedPage(reader, i));
                        }
                        copy.freeReader(reader);
                    }
                }
            }
        }

if after this piece of code I try to delete all the files in the "files" list by calling the .delete() method, it doesn't work

I also tried removing the try wth resources and manually closing reader, document copy etc... I tried inserting the PdfCopy in the try with resources together with document, but nothing, I can't find a solution...