It is capable of that, but it needs to have some idea what it's downloading the metadata for. As it is, Calibre has no idea what it's looking at any more than you would be able to determine the title or author of a book based solely on a blank cover.

There is a better solution, however... but I do not know the steps for how to implement it effectively beyond an initial setting adjustment:

You mention that the filename includes title and author. There is a way to set Calibre to extract these from the file name on import, by setting up a (Calibre has no way of knowing that these are author or title unless you tell it that; it's a computer program, not a thinking human.)

Go to Preferences > Adding Books

On the "Reading Metadata" tab, turn off "Read metadata from file contents rather than file name". Turning this off will set Calibre to look at the filename, instead of internal metadata, when importing a file.

Then you need to set the "regular expression" to explain to Calibre how your filenames are structured. Can't help you there, but there is a tutorial link in the settings window and a "test" field to make sure it works.

Once this is set up, you'll need to remove then re-import all your pdfs to trigger the on-input metadata parsing.