Complete PDF Guide
Merge, split, compress, rotate, watermark, encrypt, number, rasterise — everyday PDF work follows a pattern. This guide organises 14 operations by what they do, when to use them and what to watch out for, plus an A-series page size chart at every common DPI.
14 Common PDF Operations
| Operation | What It Does | Typical Use | Watch Out |
|---|---|---|---|
| Merge | Concatenate several PDFs in order | Contract bundles, report collections, invoice archiving | Order determines page numbers; unify page sizes first |
| Split | Break a PDF by page range or one page per file | Distributing single-page receipts, splitting scans | Ranges are 1-based, inclusive on both ends |
| Compress | Resample images and drop redundant objects | Email attachment limits, faster page downloads | Text-heavy vector PDFs have little room to shrink |
| Rotate | Rotate whole pages by 90/180/270 degrees | Fixing sideways pages from phone scans | Changes the display matrix only, not text flow |
| Watermark | Overlay translucent text or an image | Marking drafts, internal copies, copyright notices | Watermarks are removable; not an access control |
| Encrypt | Set a user password and an owner password | Statements and payslips containing personal data | Use AES-256; RC4 is considered insecure |
| Decrypt | Remove the open restriction when you know the password | Clearing your own password before bulk archiving | Only use on files you are legally entitled to open |
| Page Numbers | Add footer numbers or "Page N of M" | Tenders, theses, print-ready drafts | Mind the offset if you have a cover and TOC |
| Delete Pages | Remove chosen pages and renumber | Dropping blank scans and erroneous pages | Confirm page numbers first; the action is irreversible |
| Reorganise | Drag pages into a new order | Inserting missed pages, reordering chapters | Combine with merge and delete for full cleanup |
| Extract | Pull out text, images or selected pages | Copying clauses from a contract, exporting figures | Scanned files need OCR before text extraction works |
| To Image | Rasterise each page to PNG/JPEG at a chosen DPI | Embedding in slides, web previews, thumbnails | 72 DPI for screens, 300 DPI for print |
| Crop | Trim margins or crop to a region | Removing scan borders, unifying the text block | PDF cropping usually hides rather than discards |
| Sign | Add a handwritten or image signature; verify digital signatures | Signing contracts back, acceptance forms | An image signature is not a digital signature |
Page Sizes and Pixel Dimensions
PDF internally uses 72 DPI PDF points, so an A4 page is 595 × 842 points at native scale. The table lists three resolutions commonly used when rasterising.
| Size | Millimetres | Inches | 72 DPI | 96 DPI | 300 DPI |
|---|---|---|---|---|---|
| A4 | 210 × 297 | 8.27 × 11.69 | 595 × 842 | 794 × 1123 | 2480 × 3508 |
| A5 | 148 × 210 | 5.83 × 8.27 | 420 × 595 | 559 × 794 | 1748 × 2480 |
| A3 | 297 × 420 | 11.69 × 16.54 | 842 × 1191 | 1123 × 1587 | 3508 × 4961 |
| B5 | 176 × 250 | 6.93 × 9.84 | 499 × 709 | 665 × 945 | 2079 × 2953 |
| Letter | 216 × 279 | 8.5 × 11 | 612 × 792 | 816 × 1056 | 2550 × 3300 |
| Legal | 216 × 356 | 8.5 × 14 | 612 × 1008 | 816 × 1344 | 2550 × 4200 |
Frequently Asked Questions
Why does compressing a PDF barely reduce its size?
PDF size is dominated by embedded images, fonts and scanned bitmaps. A file that is mostly vector text with a few images simply has little to reclaim; the real weight is usually full-page scans. To shrink those you must re-export at a lower scan DPI or accept image-quality loss from image-oriented compression.
What DPI should I use when converting PDF to images?
Use 96–150 DPI for on-screen reading, embedding in slides or web previews. Use 200 DPI when small text must stay legible or the image will be re-laid out. Reserve 300 DPI for print. Doubling DPI doubles both pixel dimensions, so file size grows roughly fourfold — do not simply pick the maximum.
Does PDF encryption stop people from copying content?
Only against casual users. A user (open) password genuinely blocks anyone without it. An owner password merely sets a permission flag: most viewers honour it, but many tools ignore it outright. Do not rely on PDF permission bits to protect genuinely confidential material.
What is PDF/A and how does it differ from a normal PDF?
PDF/A is a subset of PDF designed for long-term archiving. It requires all fonts to be embedded, forbids encryption and JavaScript, and removes dependencies on external resources, so the file still renders correctly decades later. Normal PDFs optimise for editing and interactivity; PDF/A optimises for stable reproduction, and is typically required for archives, invoices and contract storage.