Hi,

You can try to use incremental merge:

http://www.essentialobjects.com/doc/eo.pdf.pdfdocument.merge_overload_5.aspx

This overloads does not do a "deep" merge as other version does and uses less memory but produces bigger files. It also requires both files to already exist on the disk. In your case, you may want to use a combined merge strategy: first perform full merges multiple times and then perform incremental merge. For example, if you have 8 files to merge, you can follow these steps:

1. Perform full merge on 4 pairs (1, 2), (3, 4), (5, 6) and (7, 8), producing 4 intermediate PDF files A, B, C and D;
2. Perform full merge on two pairs (A, B) and (C, D), producing 2 intermediate PDF files X and Y;
3. Save X and Y to disk;
4. Perform incremental merge on X and Y to produce final result Z;

The idea is to use full merge during the early stage and use incremental merge during the later stage since the benefit of incremental merge is mostly visible when the files are large.

Hope this helps. Please feel free to let us know if you still have any questions.

Thanks!