Overlap is common in PDF charts. It is a repetition of some rows and columns from the previous page. The object of this repetition is to make it easier to stitch over the page break and to find your place when switching to the next page.
Since Pattern Keeper lays out the pages next to one another the overlap is not needed and should not be displayed. Therefore PK needs to know if there is any overlap between pages and how big it is.
During import, there will sometimes be a question about this. Pattern Keeper will then show a small part of the chart and ask if there is any overlap visible and how big it is. Overlap is usually shown with some columns/rows being grey or dotted. Sometimes they can also be a light color like light blue. See below for examples of how the chart excerpt may look and the correct answer.
Has overlap: NO
Has overlap: YES
Size: 3
Has overlap: YES
Size: 2
Has overlap: YES
Size: 2