How to reduce PDF file size on Mac and iPhone without extra software

For most everyday PDF compression needs — getting a scanned form under a portal limit, shrinking an image-heavy report for email, or reducing a multi-page document for storage — the tools already on your Mac or iPhone can handle the job without any downloads.

The built-in methods work particularly well for PDFs that are large because of high-resolution embedded images. They work less well for PDFs that are already optimized or that contain mostly text. Understanding this distinction helps you choose the right approach for each file.

There are limitations. The built-in Quartz filter on Mac is effective but sometimes aggressive — it can reduce image quality more than you want. When that happens, an online tool gives you more control over the quality and compression level applied.

Preview is Mac's built-in document viewer and it has a built-in PDF compression feature through the Export with Quartz Filter option. To use it, open the PDF in Preview, go to File, then Export as PDF. In the export dialog, there is a Quartz Filter dropdown menu. Select Reduce File Size from that menu and save the file.

The Reduce File Size filter works by re-compressing embedded images at a lower resolution — typically down to 150 DPI — and applying additional compression. For image-heavy PDFs, this can reduce file size by 50 to 80 percent. For text-first PDFs, the reduction is usually much smaller because there is less image data to compress.

The main limitation of the Reduce File Size filter is that it can be too aggressive on image quality for professional documents. The output images may be noticeably lower quality than the original, which is fine for form submissions but not ideal for client presentations or work samples.

• Open PDF in Preview → File → Export as PDF → Quartz Filter → Reduce File Size.
• Works best on image-heavy and scanned PDFs.
• May reduce image quality more than desired for professional documents.
• Check the result before submitting or sharing.

The Quartz filter engine on Mac is what powers the Reduce File Size feature in Preview. It was originally designed for print workflows, and the built-in Reduce File Size filter targets 72 DPI output, which is lower than ideal for many documents. This explains why the quality reduction can feel more aggressive than expected.

Mac users who need more control can create a custom Quartz filter using ColorSync Utility (found in the Utilities folder inside Applications). This allows you to specify a higher output resolution — for example, 150 DPI — which preserves more quality while still reducing file size compared to the original.

Creating a custom filter takes about five minutes and produces much better results for professional documents. Open ColorSync Utility, go to Filters, click the plus button to add a new filter, add an Image Sampling component with your preferred DPI setting, and save the filter. It then appears in Preview's Quartz Filter dropdown.

iPhone does not have a built-in PDF compressor in the same way Mac does, but there are a few useful options. The Files app on iPhone and iPad can display PDFs but does not offer compression. The Notes app can scan documents and export them as PDFs, and the resulting files are often reasonably sized if you capture cleanly at the right settings.

For PDFs already on your device, shortcut-based workflows using the iOS Shortcuts app can route files through compression actions, but these require some setup and third-party integrations. The simplest option for most iPhone users is to use an online PDF compression tool in Safari, upload the file from Files, compress it, and save the result back.

If you frequently need to compress PDFs on iPhone, a dedicated app is the most practical option. Several well-regarded PDF utilities on the App Store offer compression, though they require a download.

The built-in Mac method works well but gives you limited control over the specific quality level applied. If you need to hit a precise file size target, preserve a specific quality level, or compress a PDF without significant image degradation, an online tool gives you much more granular control.

Built-in tools also do not handle password-protected PDFs. If a PDF has security restrictions, you will need to remove the protection before compression can work. This typically requires the original password and a PDF tool that supports unlocking before compression.

For documents where quality matters — client-facing materials, official submissions where legibility is critical, or portfolios — spending two minutes on an online compressor with quality controls is worth the extra step over the Preview filter.

If the PDF is a scanned document or image-heavy file, start with Preview's Reduce File Size filter. It often handles these cases quickly and the result is sufficient for most portal submissions. If the result looks acceptable and is within your target size, you are done.

If the Preview result is too aggressive on quality, try a custom Quartz filter at 150 DPI in ColorSync Utility. If you are on iPhone or the document requires more precise control, use an online tool. If the PDF is text-first and has barely shrunk after any of these steps, the file may already be close to its practical minimum and the content itself may need to change.

• Scanned or image-heavy PDF: start with Preview → Reduce File Size.
• Quality too low after Preview: use a custom 150 DPI Quartz filter.
• On iPhone: use an online tool via Safari.
• Text-first PDF not shrinking: the file may already be close to its minimum size.
• Need precise control: use an online tool with adjustable quality settings.