A hopper loader having at least a horizontal infeed conveyor and one or a plurality of side guides each adjustable by a single operating handle to facilitate accurate horizontal product alignment over the entire length of the hopper loader. The single operating handle drives one or a plurality of linkage assemblies which operate in unison to move a guide plate slidably engaged by side edges of printed products as they move along the horizontal conveyor sections. In applications where the hopper loader incorporates conveyor sections having different angular orientations, rods and rod sections for each such conveyor section are coupled to one another by a universal joint to convey movement to each linkage under control of the single operating handle. A side guide arrangement is also provided for use with a hopper loader having an extension coupled thereto for conveying a larger quantity of signatures, the side guide assemblies of the hopper loader and the extension being operated simultaneously through the use of a single operating handle.