You have a PDF that is too large to attach to an email, and a slider pops up asking you to choose a compression level: maximum, high, medium, or low. You want the file small enough to send, but you also want the recipient to be able to read it without squinting at pixelated text. How do you pick the right setting without guessing wrong and having to redo the whole thing?
The short answer is that for most documents being emailed, medium compression provides the best balance of file size reduction and visual quality. But the right choice depends on what is inside your PDF, who will open it, and what they will do with it. Understanding what each compression setting actually changes makes the decision straightforward.
What Each Compression Level Does to Your PDF
PDF compression works primarily by reducing the resolution and quality of images embedded in the file. Text, vector graphics, and structural elements are generally left untouched. This is why a text-only PDF compresses to nearly the same size regardless of the setting you choose. The compression level primarily controls what happens to photographs, scanned pages, logos, and other image content.
At the low compression end, images are resampled to around 150 to 200 DPI and compressed with moderate JPEG quality. Text in scanned pages remains sharp and fully legible. Photographs may show slight softening but are still perfectly usable. This setting is appropriate when the recipient needs to read fine print, examine details in photographs, or possibly print the document. The file size reduction is typically 20% to 40%.
Medium compression resamples images to around 100 to 150 DPI and applies stronger JPEG compression. Text in scanned documents remains readable, though small fonts may lose some crispness. Photographs will show visible compression artifacts if you zoom in, but they still look acceptable at normal screen viewing size. File size reduction is typically 40% to 60%. For the vast majority of email attachments, this is the sweet spot. The document loads quickly, passes through email server size limits, and looks professional when viewed on screen.
High or maximum compression pushes image resolution down to 72 to 100 DPI with aggressive JPEG compression. Text in Scanned PDF pages becomes noticeably soft, and small fonts may become difficult to read. Photographs take on a visibly degraded, artifact-laden appearance. File size reduction can reach 70% to 80%. This setting should be reserved for situations where file size is the absolute priority and visual quality is secondary, such as archiving documents you rarely access or sending preview copies where the recipient explicitly understands the quality trade-off.
How to Match the Compression Setting to Your Document Type
| Document Type | Recommended Setting | Expected Size Reduction |
|---|---|---|
| Text-only PDF, no images | Any setting (images absent) | Minimal change |
| Mostly text, a few small logos or screenshots | Medium | 30% to 50% |
| Scanned document, black and white | Medium | 50% to 65% |
| Scanned document, full color | Medium to High | 60% to 75% |
| Photo-heavy PDF, brochure, or catalog | Low to Medium | 30% to 50% |
| Legal or medical document needing exact fidelity | Low or none | 10% to 25% |
| Archive copy, rarely accessed | Maximum | 70% to 80% |
Why Email Attachment Size Limits Make Compression Necessary
Email attachment size limits are the most common reason people compress PDFs. Gmail and Yahoo cap attachments at 25 MB per email. Outlook and most corporate Exchange servers enforce the same 25 MB limit, though some organizations set lower thresholds of 10 MB or even 5 MB. If your PDF exceeds the recipient's server limit, the email bounces. You might not even receive a bounce notification. The recipient simply never sees your message.
What makes this particularly frustrating is that many PDFs cross this threshold unnecessarily. A 2025 analysis by the PDF Association found that the average scanned office document contains roughly 40% more image data than necessary for on-screen readability (PDF Association, "PDF Efficiency in Business Workflows", 2025). The extra data comes from unnecessarily high scan resolutions, color images of black-and-white documents, and embedded image formats that duplicate data already stored elsewhere in the file.
Compression strips out this redundant data without affecting the document's usability. A black-and-white scanned contract that started at 15 MB can often be reduced to under 3 MB with medium compression while remaining fully legible. The recipient can read every word, print every page, and never know the file was compressed unless they inspect the metadata.
What Happens If You Pick the Wrong Setting
The good news is that picking the wrong compression setting is not a permanent mistake. Compression is non-destructive to the original file as long as you keep the uncompressed version. If you apply maximum compression and the result is unreadable, you can go back to your original file and try again with a lower setting. The real risk is compressing a file, deleting the original, and then discovering the compressed version is not good enough. Always keep the original until you or the recipient has confirmed the compressed version is acceptable.
There is one exception to this rule. Some PDF compression tools, including older versions of certain desktop applications, apply compression destructively by overwriting the original file. Before running any compression tool for the first time, check whether it creates a new output file or modifies the original in place. If it modifies the original, make a copy first. WukongPDF's PDF Compression tool always generates a new compressed file while leaving your uploaded original untouched, so you can download the compressed version, compare it side by side with the original, and adjust settings if needed without any risk of losing your source document.
A Quick Decision Checklist
Answering three questions will guide you to the right compression setting nearly every time.
First, what is inside the PDF? If it is mostly text with few or no images, compression setting barely matters. Pick medium and move on. If it is a scanned document, medium is usually right for email. If it is a photo-heavy brochure or portfolio, lean toward low compression to preserve image quality.
Second, what will the recipient do with it? If they will only read it on a screen, medium compression is almost always sufficient. If they will print it, especially if the printed output needs to look crisp, use low compression. If they will extract images from it or use it in a publication, use low or no compression. If the document is a legal filing or medical record where image fidelity could matter, err on the side of low compression.
Third, what is your actual file size target? If you need the file under 10 MB and it is currently 12 MB, low compression will get you there. If the file is 40 MB and must fit under 10 MB, you will likely need medium or high compression. If you do not have a specific size target, medium compression is a safe default that produces good results across the widest range of document types.