Text and image watermarks are the same operation with different content, so the tools cover both and the single versus bulk split is the choice that matters:

Single file, Adobe Acrobat: Edit, Watermark, Add, offering a text watermark, DRAFT with the opacity, rotation and position controls or a file watermark pointing at your logo image or a PDF. The preview shows placement live, and the appearance options set whether it sits behind or over the content, behind being right for a logo you do not want obscuring text, a diagonal semi transparent overlay being right for DRAFT.

Free single file: Stirling PDF, PDF-XChange Editor's free tier and several others add both text and image watermarks with the same essential controls, covering the occasional need without Acrobat.

Bulk, the recurring need: Acrobat can apply a watermark to multiple files through its Action Wizard, and a pypdf script overlays a watermark PDF onto every page of every file in a folder, the scripted route giving exact repeatable placement for a standing process like watermarking every outgoing draft. The watermark itself is prepared once as a PDF or image and stamped across everything.

The two appearance decisions worth getting right: opacity, high enough to be visible and claim the document, low enough to leave text readable, usually 20 to 40 percent for an overlay and layer, over the content for a DRAFT stamp that should be unmissable, behind the content for a logo that brands without obstructing. Test on one representative page before committing a batch, since a watermark too heavy or misplaced across a hundred files is a hundred files to redo.