Batch is a queue, merge is a book
People mix these up. Batch means many jobs: file A becomes A.pdf, file B becomes B.pdf. Merge means many images become chapter pages in one PDF. Pick the verb before you pay for VPS time.
When Edge is enough
A couple of photos can still run one-by-one for free. The queue exists because fifty 12-megapixel stills will stall a phone tab.
After the ZIP lands
Open a sample. If pages look soft, the sources were already small. Compress the PDF only if a portal has a size cap.