Yes, PDFs are imported as images and connot be edited directly, but you should be able to show or hide layers in the PDF by right-clicking and choosing Object Layer Options.
Υes, this option is available in import settings as well, but it doesn't seem to make any difference. The pdf is still no editable...
Actually, no, PDF files are not placed (or to use your terminology imported) as “images” within an InDesign document where “images” refer to “raster images.”
What is true is that the content of PDF files is placed into InDesign documents without the ability to be edited. All graphical constructs (text, vector graphics, and raster images) and all attributes of same (color - including color space, rendering intents, profiles, and actual color values, transparency, fonts, resolution, shadings, patterns, line widths, etc.) other than those of PDF annotations are maintained in the place operation and depending upon output options, are either maintained or converted (in the case of color spaces and transparency for certain options).
One of the planned features of InDesign 1.0 was to allow modification of placed PDF content in InDesign. This feature was abandoned prior to the actual product release for very good reason!
The fact is that with the exception of PDF files saved from either the current or earlier versions of Adobe Illustrator using the Preserve Illustrator Editing Capabilities option and having the original fonts used installed as system fonts, opening and editing any arbitrary PDF file with Adobe Illustrator is in fact a potentially very lossy proposition! Adobe Illustrator is not, repeat is not, and repeat once again is not a general purpose PDF file editor. Adobe Illustrator does not support the complete PDF file imaging model and especially, the ability to have mixed color space documents (such as is supported by InDesign and InDesign's exported PDF files).
One of the biggest source of avoidable PDF prepress problems is the myth that Adobe Illustrator is in fact a PDF editor. Some printers routinely have problems due to workflows in which they routinely open all user-submitted PDF files in Illustrator to examine and somehow fix them up. Generally, they cause damage.
InDesign abandoned the proposed ability to edit the contents of placed PDF files because like Adobe Illustrator, Adobe InDesign also does not support the editing of all features of the PDF imaging model (it supports display, print, and export of same but not all PDF attributes have corollaries in InDesign native document objects. The user expectations of such an editing feature could not have been reasonably achieved. A very good argument could be made that Adobe Illustrator should never have allowed any editing of non-Illustrator PDF and thus prevent the ongoing myth that Adobe Illustrator is a PDF file editor.
As Peter Spier has correctly indicated, you can place PDF files with defined layers and control the participation of such layers for display or output from within InDesign. However, you cannot modify the assignment of content within such layers to other layers!
Sorry, Dov. I have to work on my wording. I should have said PDFs are imported in the same manner as images (and from a design standpoint behave the same way) and cannot be edited inside ID, but if they have layers the the visibility of those layers can be controlled.