Follow a line across the join
Inspect several pattern outlines, grain lines and registration marks across each tile boundary. A gap suggests too little overlap; a doubled or shortened segment can indicate too much overlap.
Opaque backgrounds may hide content
Later PDF tiles are painted over earlier ones in overlap areas. A white page rectangle can hide part of a neighboring tile. Trim to the intended usable region instead of relying on transparency.
Save a reversible workflow
Keep the source PDF, the chosen settings and the joined output together. Cut a cheap test pattern or toile when fit or alignment has material consequences.
Test a seam with known measurements
Cropping removes a band from each tile's visible edges before placement. Overlap then places neighboring cropped tiles closer together. These controls solve different problems: trimming an unwanted printer margin is not the same operation as overlaying duplicated pattern artwork. Measure the source instructions or registration marks rather than entering the same value for both controls by habit.
Suppose each upright source tile is 210 mm wide and you crop 5 mm from both left and right edges. Its remaining width is 200 mm. With 10 mm horizontal overlap, the next tile starts 190 mm after the previous one. Two tiles therefore cover 390 mm before any final scale factor is applied.
Inspect a continuous curve or grid line across the joined boundary at a large zoom. A doubled line suggests the overlap or source alignment is wrong; a gap can indicate excessive cropping or too little overlap. Opaque white backgrounds in a later tile can cover earlier artwork in the overlapped region, so more overlap is not always a safe fix.
The stitcher does not analyze registration marks, repair distorted scans or perform perspective alignment. Pages that were scanned at inconsistent scales need preparation elsewhere before a geometric join can be trustworthy. Save a test output and record its crop, overlap and column settings so the successful arrangement can be reproduced without relying on a screenshot of the form.