Compress a PDF on your Mac without uploading it
By Himanshu Ranjan · Updated 8 November 2026
To compress a PDF on a Mac with FileDial, select it in Finder, hold the right Option key and choose Compress › Compress PDF. Pick light, standard or strong depending on how much image detail you can give up, or a 10, 20 or 25 MB size target. Text stays selectable, a smaller copy appears beside the original, and the file never leaves your Mac.
How to do it with FileDial
- Select one or more PDFs in Finder.
- Hold right ⌥ to open the dial.
- Choose Compress › Compress PDF (light), Compress PDF or Compress PDF (strong).
- For a hard attachment limit, choose Compress › Compress under 10 MB, Compress › Email-safe (under 20 MB) or Compress › Gmail-safe (under 25 MB) instead.
- Check the result card for the before and after size, and use Undo if you want to try another level.
Your original file stays untouched. The result is saved beside it, and every result has Undo.
Which option to pick
| Tool | Use it when |
|---|---|
| Compress PDF (light)Compress | Images are kept at 200 dpi, so photos and diagrams stay sharp. Text stays selectable. |
| Compress PDFCompress | A middle setting: images at 144 dpi, fine for reading on screen. Text stays selectable. |
| Compress PDF (strong)Compress | Images drop to 96 dpi for the smallest file; text stays selectable and crisp. |
| Email-safe (under 20 MB)Compress | Squeezes images step by step until under 20 MB, and splits into parts if it can't get there. |
| Gmail-safe (under 25 MB)Compress | Same stepwise squeeze with a 25 MB target, splitting into parts if needed. |
| Compress under 10 MBCompress | Tight upload portals: squeezes images step by step until under 10 MB, splitting into parts if it can't get there. |
What makes a PDF large, and why some won't shrink
Almost all the weight in a big PDF is images. A scanned contract is really a stack of photographs of paper, and a phone scan at high resolution can make each page several megabytes. Text, by contrast, is stored as characters and fonts and takes very little space. That is why every FileDial compression level works on images and leaves the text layer alone, so you can still search and copy.
The dpi number tells you how much detail survives. 200 dpi is close to print quality, 144 dpi reads comfortably on a laptop screen, and 96 dpi is enough for on-screen viewing but small print in a scan may soften. If a PDF barely changes size, it probably has little image content to begin with: an exported text report, a form, or a file that was already compressed. Embedded fonts and attachments also add bulk that downsampling images can't remove.
- Scanned pages: expect the biggest savings from the strong setting.
- Slide decks with photos: try the light setting first so pictures keep detail.
- Text-only exports: there is little to compress, so the size may hardly move.
- A strict upload or email cap: use Email-safe or Gmail-safe, which keep stepping down until the file fits.
Why not use an online PDF compressor?
The PDFs people most need to shrink are usually the ones they must send somewhere official: signed leases, tax returns, scanned passports for a visa portal. Uploading them to a free compressor means handing a copy to a server you know nothing about. In July 2024 Cybernews reported that two online PDF services, PDF Pro and Help PDF, left Amazon S3 storage open, exposing 89,062 user-uploaded files including passports, IDs, certificates and contracts. Cybernews said the providers did not respond.
In March 2025 the FBI Denver Field Office warned that free online converter tools are increasingly used to spread malware: the conversion works, but the returned file can carry malware that leads to ransomware. FileDial compresses on the Mac itself, so there is nothing to upload and nothing to download back.
Other ways to compress a PDF on a Mac
Preview, which comes with every Mac, can shrink a PDF: open it, choose File › Export, and pick the Reduce File Size option from the Quartz Filter menu. It is free and fine for an occasional file, but it applies one fixed setting, offers no size target, and can make scans noticeably blurry.
FileDial is the better fit when you need control or volume. You choose the dpi, Compress under 10 MB, Email-safe and Gmail-safe keep going until the file actually fits under the limit, and you can select a whole folder of PDFs and run one tool on all of them. On a MacBook, a 42 MB PDF compressed in about 2 seconds. Every result has Undo, and the result card shows the size before and after.
Questions
Is compressing a PDF with FileDial free?
FileDial has a 1-day free trial with every tool, no card and no account. After that it is $20 once for a lifetime licence that works on 2 Macs, with a 14-day refund.
Does it work offline?
Yes. Compression runs on your Mac and works with Wi-Fi off. Files are never uploaded. The only network activity is the licence check and optional anonymous usage counts, which never include file names, paths or contents.
Will the text get blurry?
No. All three compression levels only downsample images, to 200, 144 or 96 dpi. Text stays as selectable text, so it remains sharp at any zoom and you can still search and copy it.
Is there a file size limit?
There is no upload limit because nothing is uploaded. Large PDFs are processed locally; the practical limits are your Mac's memory and disk space. A 42 MB PDF compressed in about 2 seconds on a MacBook.
What happens if the PDF can't get under 20 MB?
Email-safe squeezes images step by step until the file is under 20 MB. If it still can't reach that size, it splits the document into parts so each one fits. Gmail-safe does the same with a 25 MB target.
Is my original PDF replaced?
No. The compressed version is saved as a new file beside the original, which stays unchanged. Undo removes the result if you want to try a different level.