The PDF Optimization feature in Adobe PDF Library allows you to compress a PDF document to make it smaller. Besides being easier to manage, an optimized PDF document tends to load faster when opened in a web browser. The PDF Optimizer in the Library is designed to work in a similar way as a feature in Adobe Acrobat that can save a PDF document to optimize that file. Click File and Save As Other, and then select Optimized PDF.
When you optimize a PDF document, you save storage space, but it comes with a cost. The graphics images in the PDF might lose definition because resolution drops, for example. Or you could decide to remove bookmarks. You might not need these bookmarks, but they cannot be added back automatically. If your goal is to reduce the size of a PDF document from, say, 8 MB to 1.5 MB, so that it can be more easily sent out as an attachment to an email message and allow for faster paging online, optimizing that document makes sense. But if providing high-quality graphics for printing is more important, the document should not be optimized. And if your goal is to archive a PDF document so that it will be a useful reference 20 years in the future, optimizing it might be a mistake, as the graphics will be harder to read, and saving storage space might not be a priority.
Note also that the Optimizer in the Adobe PDF Library saves file output as standard PDF files, not as PDF/A. The Library offers a separate PDF/A conversion feature.