Images are often the biggest reason
Scanned PDFs and photo-based documents can be large because each page contains image data. High-resolution scans, color photos, and repeated images add up quickly.
A PDF that looks simple on screen can still contain large images behind the page.
Fonts and structure can add weight
PDFs can embed fonts, thumbnails, metadata, forms, and internal object streams. Some of this is useful, and some may be optimized depending on how the file was created.
Lossless compression can improve compatible PDFs, but it cannot guarantee major savings for every file.
What to try
Try Compress PDF for structural optimization. If the file is built from images and remains large, the source images may need resizing or recompression before creating a new PDF.
Related ConvertZiva tools
FAQ
Why did compression barely change my PDF?
The PDF may already be optimized, encrypted, damaged, or mostly made of image data that the current lossless tool does not downsample.
Does deleting pages reduce size?
It can, especially when removed pages contain large images, but file size depends on the whole PDF structure.