I'm trying to make a portlet which shows most downloaded files from document library.
Thanks in advance.
Take a look at com.liferay.portlet.documentlibrary.service.DLFileEntryServiceUtil
Specifically
com.liferay.portlet.documentlibrary.service.DLFileEntryServiceUtil.getFileEntry(long groupId, long folderId, java.lang.String name)
You will get DLFileEntry so
DLFileEntry dlfe = com.liferay.portlet.documentlibrary.service.DLFileEntryServiceUtil.getFileEntry(my-group-id, folder-id, name);
dlfe.getReadCount();
will get you what you want.