These are the tools you reach for when a file is fine in content and wrong in some other way: too heavy for an upload limit, printing badly because half its pages are US Letter, or refusing to open at all.
Compression is the one with the most marketing noise around it, so it is worth stating plainly: most of a PDF's weight is images, and a document without images will not shrink much no matter what any tool claims. This one reports the real before and after every time, including when the honest answer is 'almost nothing'.
Which one do you need?
The file is too large to upload
Compress PDF recompresses embedded images and rebuilds the file structure. Start at balanced and read the reported saving. If the document is a phone scan, going back and rescanning in greyscale with Scan to PDF beats compressing a colour photograph twice.
It prints with inconsistent margins
Resize PDF scales every page proportionally onto one paper size. This is the fix for a merged document whose sources disagreed about A4 versus US Letter.
It will not open
Repair PDF rebuilds a damaged cross-reference table, which is what an interrupted download usually breaks. It cannot recover data that never arrived, and it says so rather than returning a file that silently lost pages.
Form entries vanish for other people
Flatten PDF draws live form fields and annotations into the page so every reader and every printer shows them. Do this before submitting any PDF cover sheet.
At a glance
| Tool | What it changes | Reversible? | Typical use |
|---|---|---|---|
| Compress PDF | Image resolution and quality | No - keep the original | Clearing an upload size limit |
| Resize PDF | Physical page dimensions | No | Printing a mixed-size document |
| Repair PDF | Internal file structure | n/a | A download that will not open |
| Flatten PDF | Fields become page content | No | Submitting a completed form |
Before you keep the result
- After compressing, open the busiest page and read the smallest text before keeping the result.
- A saving under about ten per cent means the document was never image-heavy. That is information, not failure.
- Resizing drops links and form fields, because each page is redrawn onto a new sheet.
- Flattening cannot be undone. Keep the editable original.
Questions and answers
Why did compression barely shrink my file?
Because its weight was text and vector drawing rather than photographs. There is nothing image-shaped to remove. A tool claiming a large reduction on such a file is rasterising your text, which makes it blurry and unsearchable.
What is the smallest I can get a scanned document?
Scan to PDF at the black-and-white setting produces by far the smallest files, because a one-bit image of text compresses extraordinarily well. Check faint pencil survived before you rely on it.
Can a repaired PDF be trusted?
Check it page by page against what you expected. A repair restores structure, not certainty about content.