Zotero automatically renames downloaded files based on the parent item metadata. In most cases, you don't have to do anything, or you should correct the parent metadata and then use Rename File from Parent Metadata to update the filename.
If you want to choose a different name, you can click on the attachment title in the right-hand pane after clicking on the attachment and choose "Rename associated file". (That will change the attachment title too, but in the upcoming Zotero 7 there will be separate options for renaming the attachment title and the filename.)
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If you want to choose a different name, you can click on the attachment title in the right-hand pane after clicking on the attachment and choose "Rename associated file". (That will change the attachment title too, but in the upcoming Zotero 7 there will be separate options for renaming the attachment title and the filename.)