Rotate PDF Pages

Rotation in a PDF is a Rotate entry holding a multiple of 90 degrees. It changes how a viewer presents the page without touching the content stream, so rotating is cheap and the coordinates of everything on the page stay where they were. Two places can hold the entry, and which one you write to decides whether it takes effect.

The per-page form is the one that always works. Reach for SetRotation only when you know the source document has no rotation set anywhere.

Doing it

degrees := int64(90)

pdfWriter, err := pdfReader.ToWriter(&model.ReaderToWriterOpts{
    PageProcessCallback: func(pageNum int, page *model.PdfPage) error {
        page.Rotate = &degrees
        return nil
    },
})
if err != nil {
    return err
}

err = pdfWriter.WriteToFile(outputPath)

To rotate a single page, compare pageNum inside the callback; it is 1-based. To add to the existing rotation rather than replace it, read page.GetRotate() first, since page.Rotate is nil on a page that inherits its rotation.

Limitations

The global rotation loses to any per-page rotation, and ToWriter creates per-page rotations. PdfWriter.AddPage copies the inheritable attributes - Resources, MediaBox, CropBox and Rotate - down from the page tree onto each page dictionary, so after ToWriter every page carries an explicit Rotate if the source declared one anywhere in its chain. A later SetRotation on the writer then has no effect on those pages.

Concretely: take a document whose root Pages node has Rotate 90, run it through ToWriter and call SetRotation(180). The output has Rotate 90 on each page and Rotate 180 on the Pages node, and the effective rotation is 90. Nothing reports an error. This is the failure mode behind “rotation did not change anything”.

Degrees must be a multiple of 90. UniPDF does not validate this - SetRotation writes whatever integer you give it - so the check belongs in your code. page.Size() only swaps width and height when the value is a multiple of 90, and viewer behavior on other values is undefined.

Rotation is presentational. Text extraction, annotation rectangles and content stream coordinates are all unaffected, which is usually what you want and occasionally not. To push the rotation into the content so that the origin ends up at the visual top left, see flatten page rotation.

Sample input

Run the example

rotatePdf takes an input path, an angle and an output path, and applies the angle globally with pdfWriter.SetRotation. Read the limitations above before using it on documents of unknown provenance. For the per-page approach, the examples repository has pdf_page_rotate.go, which sets page.Rotate from inside the callback.

git clone https://github.com/unidoc/unipdf-examples.git
cd unipdf-examples/pages
go run pdf_rotate.go input.pdf 270 output.pdf

If this is your first time using UniPDF, follow the getting started guide to create an API key and set up your development environment.

Sample output