Over the past few years, Bluebeam has received more attention and I have many colleagues who think its great b/c they can mark-up pdfs on the fly and it tracks the changes for delivery back to the consultant. For example, they can snap part of the design, shift or mirror it, and paste it elsewhere; a kind-of easy 2D editor. They can measure things and so forth. For them, its a new tool. This is especially true if they do not have CAD or BIM software, such as various non-A&E project team members.
I've used similar features via Adobe Acrobat Pro (albeit not as extensive a tool kit for architecture). But the idea is the same - quick mark-ups with tracking = speed of on-the-fly mark-ups (easy in Skype meetings) and accountability for where the changes come from.
That said, though my office provides Bluebeam to me and I've used it a few times, I wholeheartedly agree that live ArchiCAD editing far out-distances the 'pdf mark-up strategy' for design process. Having a portable virtual model (in BIMx) off to the client beforehand and then in-person screen or Skype calls with a skilled AC-based architect is a far more effective process. As we know, it provides so much more visualization and single point of editing = quality.
Also of note, many offices are using Revit, which is heavier and slower on-line than AC (according to one of our architects who knows both platforms). Just last month they held Skype sessions with the client on a fully equipped 50,000 SF project. They still relied heavily on Sketch-Up and Bluebeam b/c the Revit model was too slow to edit very much while on-line. (It may be a good idea to run some tests if you are routinely working with larger or more complex models.)
Hope this helps.
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