Compressing a PDF for email: attachment limits, image weight and what actually shrinks
Large PDFs are usually not one problem but several: high-resolution photos, scanned pages, embedded fonts and leftover metadata all add weight. Compressing a PDF is about finding the biggest contributors and reducing them, in the right order.
This guide explains why PDFs get large, what compression actually changes, and how to use the Paperly Compress PDF tool to get the smallest honest result for your document. The tool runs entirely in your browser — your file never leaves your device.
Why PDF files get so large
A PDF is a container. Its size is the sum of the objects inside it: page descriptions, text, images, fonts and metadata. In practice, four things dominate file size:
- High-resolution images — a single 300 dpi full-page scan or a large photograph can exceed the weight of hundreds of pages of text.
- Scanned pages — a scanned PDF is an image PDF. Every page is a picture, and pictures do not compress like text.
- Embedded fonts — PDFs often embed full font files (some are 1 MB or more each) even when only a few characters are used.
- Leftover metadata and duplicate resources — author names, timestamps, and copies of the same image stored repeatedly across pages add redundant bytes.
Understanding this matters because it tells you what to expect. A text-only PDF built from a word processor is already small; compressing it rarely produces dramatic savings. An image-heavy PDF with scanned pages and photos is where compression earns its keep.
Lossless vs lossy compression
Compression comes in two broad families. The distinction decides how much you can shrink a file and what you give up:
| Approach | What changes | Quality impact | Best for |
|---|---|---|---|
| Lossless | Re-encodes existing data without discarding information | None — content is byte-identical after decompression | Text PDFs, documents with vector graphics, legal archives |
| Lossy | Discards detail the eye tolerates (e.g. JPEG quality, resolution) | Some visible difference at strong settings | Scanned pages, photos, image-heavy documents |
Most practical PDF compression is a mix. Text and structure are re-encoded losslessly; images are the part where a lossy step produces the real size reduction. The art is choosing how aggressive the lossy step should be for your use case.
How compression works in the Paperly tool
The Paperly Compress PDF tool offers two modes, and the difference is important to understand before you run it:
Standard compression
Standard mode re-encodes the document structure and image streams while keeping the text layer intact. Text remains selectable, searchable and copyable — which matters for contracts, reports and any document people need to quote or search. The size reduction is real but modest on already-tight files.
Maximum compression
Maximum mode renders every page to an image at a reduced quality and rebuilds the PDF from those images. This produces the smallest output for image-heavy documents, but the original text layer is gone: text is no longer selectable or searchable in the result. The interface warns about this before you process, and you should only choose Maximum when keeping text selectable is not important.
| Mode | Text stays selectable | Size reduction | Typical use |
|---|---|---|---|
| Standard | Yes | Moderate | Sharing documents that others must search, quote or edit |
| Maximum | No | Largest | Emailing scans or photos where only the visual content matters |
How to compress a PDF step by step
- Open the Compress PDF tool.
- Upload the PDF you want to shrink (up to 100 MB, processed locally).
- Choose a mode: Standard keeps text selectable; Maximum shrinks furthest but removes the text layer.
- Click Process and wait for the progress indicator to finish.
- Review the result — the tool shows the before and after size so you can judge whether the trade-off was worth it.
- Download the compressed PDF, or start over with the other mode and compare.
Zipping vs compressing a PDF: they are different
A common question is why you cannot just put the PDF in a ZIP file and call it compressed. The two operations work at different levels:
| Approach | How it works | Result | Best for |
|---|---|---|---|
| ZIP (or any archive) | Losslessly compresses the whole file as one byte stream | The PDF inside is byte-identical | Transport and archival, not sharing a single smaller PDF |
| PDF compression | Re-encodes the content: images, fonts and structure | The PDF itself is smaller and opens directly | Emailing or uploading a smaller document |
A ZIP of a PDF is still a ZIP — the recipient must unzip it to read the document. PDF compression makes the document itself smaller so it opens anywhere without an extra step. For text-only PDFs the two can produce similar savings; for image-heavy PDFs, real PDF compression goes further because it re-encodes the images lossily, which ZIP cannot do without breaking the file.
Image resolution and DPI: where the size really lives
Image weight scales roughly with the number of pixels. A page scanned at 300 DPI has about four times the pixels of the same page at 150 DPI — and roughly four times the weight before compression. That is why two scans of the same document can differ wildly in size.
| Purpose | Typical resolution | Notes |
|---|---|---|
| On-screen reading | 96–150 DPI | Looks fine on screens; much smaller files |
| Archiving / documents with fine print | 200–300 DPI | Better legibility when zooming or printing |
| Print reproduction | 300 DPI and above | Overkill for most everyday sharing |
When a PDF is built from images, the source resolution is baked in at creation time. Compressing an already high-resolution PDF re-encodes those pixels; reducing the resolution itself (when you create the PDF) is the bigger lever. On Paperly, Maximum mode effectively does this for you by rendering pages at a smaller size — which is why it shrinks scanned documents the most.
Which compression mode should you use?
There is no single right answer — it depends on what the recipient needs to do with the file:
- Submitting a report or contract: use Standard. The recipient will search or quote it, and selectable text is worth more than a few hundred KB.
- Emailing a scan or photo album: use Maximum. The visual result is what matters, and the smallest size clears email attachment limits.
- Archiving documents you will rarely open: use Maximum for the smallest footprint, but keep the original elsewhere if you may need the text layer later.
- Unsure: run Standard first. Check the size. If it is still too large and text selectability does not matter, run Maximum and compare.
Realistic expectations
The honest rule: results depend on the document. The examples below are illustrative, not measured promises:
| Document type | Typical result | Why |
|---|---|---|
| Text-only PDF (10 pages) | Small change | Text was already compact; there is little to remove |
| Scanned PDF (image pages) | Large reduction in Maximum mode | Page images are the whole file, so re-encoding them is everything |
| PDF with embedded photos | Large reduction | Photographs dominate size and tolerate quality reduction |
| PDF with many embedded fonts | Moderate reduction | Font data can be re-encoded or subset |
If a compressed PDF is barely smaller than the original, the file was probably text-only or already optimized — that is normal, not a failure of the tool.
Common mistakes
- Compressing repeatedly — compressing an already-compressed file gains almost nothing and can degrade images with each pass. Compress once, from the original.
- Using Maximum on text documents that others must edit — the text layer is gone and the document becomes an image, which is not editable as text.
- Expecting a fixed percentage — any site that promises a guaranteed ratio is oversimplifying. Image-heavy PDFs shrink far more than text PDFs.
- Ignoring what makes the file large — compressing a 40 MB file full of photos with Standard mode and then concluding compression "does not work" misses the point: the images need the lossy step.
- Storing only the compressed copy — if you used Maximum, keep the original elsewhere in case you need searchable text later.
Troubleshooting
The compressed PDF is still large
Likely causes: the file is text-only with little to remove, or the images are the whole file and Standard mode left them intact. Try Maximum mode, and check the before/after sizes to confirm what changed.
Text is no longer selectable after processing
You used Maximum mode — it rasterizes pages by design. Re-run with Standard mode if you need selectable text.
The output looks noticeably lower quality
Maximum mode trades quality for size. If the reduction was more than you needed, use Standard mode or accept a smaller reduction — the tool lets you compare both.
The tool reports an error with my file
The file may be corrupted or use unsupported features. Check that it opens in a normal PDF reader, then try again. Because processing is local, you can also try a fresh copy of the file.
Privacy: where compression happens
Compression on Paperly is 100% client-side. The PDF is read, re-encoded and written entirely inside your browser tab — the bytes never leave your device, so there is no upload, no server copy and nothing to delete. This matters for confidential documents: no third party sees the file while it is being compressed.
Limitations
- File size limit: 100 MB per PDF — a browser-memory limit, not a paywall.
- No guaranteed compression ratio — results depend on the document content.
- Maximum mode removes the text layer (text becomes non-selectable).
- Very large documents may take longer to process, since rendering pages to images is compute-heavy in the browser.
- The tool compresses the PDF you give it; it does not re-scan or improve source images.