downloaded file not in default download folder even can't find them

[ghost]

i found that the downloaded file include homework, file in announcement, or teaching material is not in the default folder.
i even look up the file in my cellphone but failed.
I think the downloaded files were put in to the system ROM so that we cannot find them.
Furthermore, the file name is not the name that was uploaded.
For instant, if i download a pdf file, the name will be "downloadfile.pdf".

[geniusgordon]

Do you have notification when downloading?
I am using the Andoird's system download manager, files will be saved in the system Downloads folder, chrome saves download files in the same place

[ghost]

when i click the file, the file will be download and the notification shows that the file of original name was downloaded completely.
i can open the file by click the notification of download complete.
but if i want to open the file i download before, i cannot find them in the download folder.
though i can still open the file by download it again but it doesn't work when i am out of internet.

my cellphone is samsung galaxy s7 edge, and my android version is 6.0.1
if necessary i can record and upload my screen to show what happen