Canon PIXMA iP8750 - Manual (Page 63)

3. Pull out the paper output tray.
4. Slide the paper guides (B) to open them, and load the paper in the center of the rear tray
WITH THE PRINT SIDE FACING YOU.
The folded flap of the envelope will be faced up on the left side.
Up to 10 envelopes can be loaded at once.
5. Slide the paper guides (B) to align them with both sides of the envelopes.
Do not slide the paper guides too hard against the envelopes. The envelopes may not be fed properly.
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