@san2710: It sounds like you are trying to "push a rope" with these files.
PDF has its limitations. It is an antique (read: dates from before the Internet) format and was not designed to be an ebook format. It lacks any true support for "covers", at least in the sense we use with ebooks. Trying to massage them so that they automagically import into a new Calibre library with all the data you want will not be easy and some things might not be possible.
Since your intent is to distribute these files as a Calibre Library and you already have the books in your main Calibre Library it would seem to make more sense to:
- In your Calibre installation, create a new empty library at some convenient location.
- Switch back to your main library (the creation step switches you to the new library)
- Select the books you wish to distribute.
- Click the "Copy to library" button on the tool bar (this may not be present by default; if so, go to Preferences>Toobars>Main Toolbar to add it).
- Select your new empty library from the menu.
- Switch to the new library and test it to be sure you've created it correctly.
- Switch back to your regular library
- Exit Calibre
- Using Windows File Explorer (or macOS Finder), copy the new library folder and all of it contents into the Calibre Portable folder tree on the USB flash drive placing it beside the existing "Calibre Library" folder.
You could, if you wish use the "Calibre Library" on the USB flash drive as the target of Step #4, skipping the creation of a new empty library. If you do, just be careful to avoid confusion with your main library as both will likely have the same names. Also, once you've copied the new exported library into the Calibre Portable folder tree, you could delete the "Calibre Library" that is alread there and rename the folder for the new exported library the exactly the same name. Calibre Portable always uses "Calibre Library" as its default library when launched; this can't be changed.