Can not upload PDF file to Firebase Storage - ios

Question

I need to upload a PDF file to Firebase Storage, and I works with my Simulator. But when I test on a real device, I get an infinite loading loop...
I searched for solutions, and read that I have to putData instead of putFile, and add a metadata.contentType.
I followed these 2 steps, but unfortunately it still does not work on a real device...
Here's my function:
func uploadDocs(docsArray : [URL], completionHandler: #escaping ([String]) -> ()){
var uploadedDocsUrlsArray = [String]()
var uploadCount = 0
let docsCount = docsArray.count
for doc in docsArray{
let docName = UUID().uuidString
let storageRef = Storage.storage().reference().child("documents").child(docName)
let metadata = StorageMetadata()
metadata.contentType = "application/pdf"
if let data = NSData(contentsOf: doc) as Data? {
storageRef.putData(data, metadata: metadata) { (_, err) in
if err != nil {
print(err!.localizedDescription)
return
}
storageRef.downloadURL { (url, err) in
if err != nil {
print("Failed to download url:", err!)
return
} else {
uploadedDocsUrlsArray.append(url?.absoluteString ?? "")
uploadCount += 1
print("Number of documents successfully uploaded: \(uploadCount)")
if uploadCount == docsCount{
print("All documents are uploaded successfully")
completionHandler(uploadedDocsUrlsArray)
}
}
}
}
}
}
}