Editing PDFs in the browser: what merging, splitting and converting keep
A PDF describes pages exactly: every line of text, image and font sits at a fixed position, which is why the file looks the same on any device. Tools that merge, split, rotate or convert PDFs work on those pages. What happens to everything around them, such as bookmarks, links between pages, form fields and the document’s own information, depends on the operation, and some of it is lost. This guide explains what each operation keeps, how page sizes and image resolution work, what a PDF’s metadata can reveal, and why covering text is not the same as removing it. The behaviour described is that of the nTools PDF tools, which were tested on a sample file for this guide; other software can differ.
How a PDF measures a page
PDF sizes are given in points, where one point is 1/72 of an inch, or about 0.353 mm. A page is simply a width and a height in points:
| Page size | Millimetres | Points |
|---|---|---|
| A4 | 210 × 297 | 595.28 × 841.89 |
| A5 | 148 × 210 | 419.53 × 595.28 |
| US Letter | 215.9 × 279.4 | 612 × 792 |
| US Legal | 215.9 × 355.6 | 612 × 1008 |
Because the size belongs to each page, one PDF can mix sizes and orientations, and merging an A4 document with a Letter one keeps both as they are.
What merging, splitting and rotating keep
Merging and splitting build a new PDF and copy the pages into it. The pages themselves arrive intact, but some things that belong to the document rather than to a page do not. Rotating only changes a setting on each page and saves the document as a new file, so it keeps everything in the table, although, as after any change, a digital signature on the original is no longer valid:
| Part of the file | Merge | Split, extract, reorder | Rotate |
|---|---|---|---|
| Page content, with selectable text | Kept | Kept | Kept |
| Links to websites | Kept | Kept | Kept |
| Links to other pages of the document | Stop working | Stop working | Kept |
| Bookmarks | Lost | Lost | Kept |
| Fillable form fields | Values visible; the form may stop working | Values visible; the form may stop working | Kept |
| Title, author and other document information | Not carried over | Copied from the original | Kept |
If a document relies on bookmarks or a fillable form, keep the original and send the merged file as a copy for reading, or merge it with software that rebuilds bookmarks and forms.
Rotating without losing quality
A PDF page has a rotation setting of 0, 90, 180 or 270 degrees, and viewers apply it when they display or print the page. Rotating a page changes only that setting: nothing is redrawn or recompressed, the text stays selectable, and rotating the same page four times by 90 degrees returns it to where it started. A rotation is added to the one the page already has, so a scanned page that opens sideways is fixed by rotating it once in the right direction.
Images to PDF: how the pages are built
A JPEG photo is placed in the PDF exactly as it is, byte for byte, so there is no second round of compression and no loss of quality. A PNG image is stored with lossless compression, although 16-bit images are reduced to 8 bits per channel. The resulting PDF is therefore about as large as the images put together; to make it smaller, reduce the images first.
On an A4 or Letter page, each image is scaled to fit inside a 24-point margin, about 8.5 mm, and, with automatic orientation, the page is turned to landscape for images that are wider than they are tall. Phone photos are often stored sideways with a tag that says how to turn them; the tool reads that tag and places each photo the right way up, still without recompressing it. With the original size option, each pixel becomes 0.75 points, the usual 96 pixels per inch on screen, so a 1200 × 900 pixel image makes a page of 900 × 675 points, about 31.8 × 23.8 cm.
PDF to images: choosing a resolution
Converting the other way draws each page as a picture. The resolution sets how many pixels each inch of the page gets:
| Resolution | A4 page in pixels | Good for |
|---|---|---|
| 72 DPI | 596 × 842 | Thumbnails and quick previews |
| 108 DPI | 893 × 1263 | Reading on screen |
| 144 DPI | 1191 × 1684 | Sharper text and zooming in |
PNG keeps every pixel exactly and suits text and diagrams; JPEG makes smaller files and suits photos, at the quality you choose. In both cases the text becomes part of the picture: it can no longer be selected, copied or found with a search. Up to 20 pages can be converted at a time.
What a PDF’s metadata says
Most PDFs carry a short record about themselves, the document information: a title, author, subject and keywords, the application that created the original, such as a word processor, the software that produced the PDF, and the dates it was created and last modified. It is invisible when reading, but anyone can display it, and it often contains a person’s name or an organisation’s software.
Files can also carry a second, longer block of metadata in a format called XMP. The nTools metadata viewer shows the document information, which is what PDF viewers usually show under the document’s properties, but not the XMP block, which can hold the same names. Merged, split and extracted files do not carry the XMP block over; a rotated file keeps it.
Hiding text is not removing it
Drawing a black rectangle over text, or changing its colour to white, only covers it. The text is still in the file: in the sample tested for this guide, a number hidden under a black box came straight back with an ordinary copy and paste. To remove information, use a proper redaction function, which deletes the content underneath, or convert the pages to images and back to a PDF, which keeps only the pixels; the text is then no longer searchable, and the new file does not carry the original’s document information.
Limits and privacy
- Every nTools PDF tool runs in your browser: the files are read and written on your device and never uploaded.
- A single PDF can be up to 80 MB, with 150 MB in total per operation; images can be up to 30 MB each and 80 MB together. Splitting creates up to 30 files at a time.
- Password-protected PDFs, and PDFs locked against printing or editing, cannot be opened by these tools. Remove the password in the application that set it first, if you are allowed to.
Frequently asked questions
Why did my bookmarks disappear after merging?
Bookmarks belong to the document, not to the pages, and merging builds a new document from the pages. The same happens to links that jump to another page and to fillable forms.
Does rotating a PDF reduce its quality?
No. Rotation only changes a setting on the page; nothing is redrawn or recompressed.
Why can’t I select the text after converting pages to images?
Because the text has become pixels in the picture. Keep the original PDF if you need the text.
Are my files uploaded?
No. All the nTools PDF tools process files in your browser, on your own device.
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