Adobe Portable Document Format (PDF) is a versatile file format that can represent both vector and bitmap data. You can bring artwork from PDF files into Illustrator using the Open command, the Place command, the Paste command, and the drag-and-drop feature. Use the Place command with the Link option selected to import a PDF file (or one page of the PDF if it is a multi-page document) as a single image. You can modify the linked image using transformation tools, however, you cannot select and edit its individual components. Use the Open command or the Place command with the Link option deselected to edit the contents of a PDF file. Illustrator recognizes the individual components in the PDF artwork and lets you edit each component as a discrete object. Use the Paste command or the drag-and-drop feature to import selected components from a PDF file, including vector objects, bitmap images, and text. Note: If you are color-managing artwork in a document, embedded PDF images are part of the document and therefore color-managed when sent to a printing device. In contrast, linked PDF images are not color-managed, even if color management is turned on for the rest of the document. When you place an Adobe PDF file, you specify which page you want to import. You also choose how to crop the artwork by selecting a Crop To option: Places the PDF page’s bounding box, or the minimum area that encloses the objects on the page, including page marks. Places the PDF only in the area defined by a rectangle that the author created as placeable artwork (for example, clip art). Identifies the place where the final produced page will be physically cut in the production process, if trim marks are present. Places only the area that represents where all page content should be clipped, if a bleed area is present. This information is useful if the page is being output in a production environment. Note that the printed page may include page marks that fall outside the bleed area. Places the area that represents the physical paper size of the original PDF document (for example, the dimensions of an A4 sheet of paper), including page marks. When you import artwork from an Adobe PDF file, it’s possible to introduce data that you can’t create within Illustrator. This is called non-native art and includes monotone, duotone, and tritone images. You can also generate non-native art within Illustrator by using the Flatten Transparency command to preserve spot colors. Illustrator’s ability to preserve non-native art is useful in many situations. For example, Illustrator maintains the spot color information in linked PDF files when you output color separations. By default, non-native art is labeled <,Non-Native Art>, in the Layers and Appearance panels. You can select, move, save, and perform basic transformations (such as scaling, rotating, or skewing) on non-native art. However, you cannot select and edit its individual components. In addition, you must rasterize non-native art before editing it with the liquify tools. Source.