Why doesn't the president sign the full bill at the ceremony?

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It is apparent to me that the president does not sign the full bill at the signing ceremony. Obama's health care overhaul, for example, is 1017 pages long. Based on that, one would expect to see something the size of a college textbook on the table during the signing ceremony. See, for example, the picture of Lyndon Johnson signing the Medicare bill. Instead, we see only a slim folder containing no more than two pages. This entry should identify exactly what the president is signing at the ceremony, if not the full bill, and provide a photo, if possible, of what is signed. Apparently, there was a time when the president signed the full bill at the signing ceremony. The entry should also explain when, why, how, and by whom the ceremony was changed so that the president now signs only a two-page document.