What does PDF compression do, and how much smaller can a PDF get?
PDF compression reduces the file size of a PDF by re-encoding its embedded images at a lower quality or resolution, while keeping text, vector graphics, and document structure fully intact. On iPhone, PDF compression typically shrinks a scanned document by 35–70% depending on the chosen quality level: Maximum reduces size by about 15%, High by 35%, Medium by 55%, and Low by up to 70%. ScanLens runs compression on-device, so the original file never leaves your iPhone during processing.
Compression solves a size problem, not a page-selection problem. If the PDF is already correct but too heavy to email, upload, or store efficiently, this is the right tool.
If you need to combine files first, use Merge PDF. If you only need certain pages, use Split PDF. Compression is for keeping the same document while making the file smaller.
That is why this page targets attachment and upload intent: reducing file size without rebuilding the document from scratch.
Compression also pairs well with the handoff stage of a workflow: prepare a signed file for delivery with PDF signing, add password protection, or send the lighter version through your scan-to-cloud workflow.
Downsampling and the quality-vs-size trade-off, honestly
There are two levers behind every quality level, and it's worth knowing what they actually do to your file. The first is re-encoding: the embedded image is saved again at a lower JPEG quality, throwing away fine detail the eye barely registers to make the file smaller. The second is downsampling: the image's pixel resolution is reduced — a page scanned at 600 DPI doesn't need 600 DPI to look sharp on a phone screen or in an email, so the resolution drops to something like 150–200 DPI and the pixel count falls sharply.
Both levers are why scans shrink so much. A high-resolution capture of a paper page carries far more pixels than any screen will ever show; trimming that surplus is mostly free in visible terms. The trade-off only becomes noticeable as you push toward the heaviest settings, where edges of small text soften and large photos can show blocky artifacts.
This is genuinely lossy compression: the pixels removed to shrink an image cannot be reconstructed from the smaller file. That's not a flaw — it's how meaningful size reduction works for images — but it sets a clear expectation. There is no setting that makes a file dramatically smaller and pixel-for-pixel identical. The skill is choosing the lightest touch that still meets the need: enough reduction to clear the limit you're fighting, no more.
Compression vs the original scan — keep both
It helps to separate two different things people sometimes blur together. Scan quality is fixed the moment you capture a page: the lighting, the focus, the resolution of the original scan. Compression happens afterward and can only ever subtract from that — it reduces the file size of whatever quality you already have. Compressing will never make a blurry scan sharper; it can only make a sharp scan into a smaller, slightly-less-sharp file.
Because compression is lossy, the right habit is to treat it as a delivery step, not an editing step. ScanLens compresses into a new copy and leaves your original PDF in the library untouched. Keep that original. If you compress a contract scan down for email today and a client asks for a print-resolution copy next month, you re-export from the pristine source rather than trying to rescue detail from the compressed version — which isn't possible.
The same logic explains why a text-based PDF barely shrinks while a scanned one collapses to a fraction of its size. Compression acts on images; in a scanned document nearly all the bytes are images, so there's a lot to remove. In a born-digital text PDF the characters are already compact, so even Low quality has little to work with. Match your expectation to the kind of PDF you're holding, and the result will rarely surprise you.
