You can make the very long image using photoshop. I just tried with a sample PDF of 10 pages.
1) Open your document in Photoshop. When the dialog appears as to which page you want to open, hold down the shift key and select all the images.
2) Your pages are now all individual documents on screen right? How many pages did you open? This is important. Next go under the File Menu to Automate. Select Contact Sheet II. A dialog will appear.
To make it easier on yourself set the units pulldown to pixels. Set the width number to the dimensions of project’s width (768, 720, 1280, 1440, etc). Check the Flatten all layers if you just want one document in the end. In the thumbnails section set the columns to 1 and the rows to the number of pages your PDF generated into documents for Photoshop. Now watch the little example on the side and adjust the number of pixels in height so you have only one document created (number of pages * comp width). You should see Page 1 of 1.
Make sure use filename as caption is turned off. Turn off the auto spacing as well and set the vertical and horizontal to center, so all your pages tile. Set you document resolution to 72.
3) Save document as PSD, TIFF, etc and import into After effects.
I hope the rest is self explanitory.