Why some PDFs compress and others do not
A PDF is a container that can hold text, fonts, vector graphics, raster images and streams already stored in compressed form. Depending on its contents, optimizing the document structure may reduce overhead or remove some inefficiency.
A PDF dominated by large photos is different from a mostly text-based PDF. If those photos are already compressed, lossless structure optimization alone may make only a small difference.
Check the result before sharing
- Note the source PDF file size.
- Run Compress PDF and download the output.
- Compare the file sizes and open the result.
- Inspect pages, embedded graphics and text legibility before submitting it.
Compression is not always image downsampling
Some compressors recompress embedded pictures at lower quality. Others optimize PDF objects without reducing image resolution. FileVado's Compress PDF tool does not promise a target size or aggressive image downsampling.
If you need to reduce a PDF below a strict size limit, you may need to resize large source images or export a smaller PDF from the original editor.
What to try when compression is not enough
If the PDF came from a slide deck or design application, export it again with image compression appropriate for the destination. When the document contains scanned photographs, creating a new source with lower image resolution may reduce size more than optimizing an already-compressed PDF container.
Do not aggressively reduce resolution if text, barcodes or fine diagrams must remain legible. Always open the compressed document before submitting it, especially when it contains pages that must be printed.