What does flattening of a PDF mean for printing?

[xpertpse]

I have created a PDF with images in it. The images are on different layers in PDF. PDF also contains some layers which have some text on it. The images are of the required resolution (>300 DPI) for printing. Also, some layers have transparency in it (want to make the background layer visible).

But when I send that PDF to a print office, it says flatten the PDF and remove transparency. I have the following questions about flattening the PDF:

1. What does flattening the PDF mean? (I suppose make it a single layer document)
2. After flattening the PDF, would I loose the resolutions of the images and what would the final resolution be?
3. Will the PDF be transparency free after flattening?
4. What are the things that I will loose after flattening the PDF (what are the limitations of the flattened PDF)?
5. How to flatten a PDF?

I have tried to find the answers of these questions by myself, but I think I need some PDF expert for these questions. Please help me out.

Thanks in advance.

[Test Screen Name]

Flattening means several quite different things for PDFs. At least two of them apply specifically to your case, and with printing there is too much at stake to guess; I would advise you to ask the following question:

"We've been told that flattening could mean

- removing transparency effects

- combining layers into one

- making sure comments and form fields are part of the background (probably not relevant).

Please let me know which kind(s) of flattening you need - transparency and/or layers.

Also, do you need a particular version of PDF (e.g. PDF 1.4) or of PDF/X (e.g. PDF/X-3)? It would be helpful to know why you have this requirement."

The further questions are important too, as they usually go together.

TO answer specifically

1. See above

2. From flattening transparency, potentially, settings must be chosen carefully.

3,4. Depends on your choice.

5. Depends what you need. If it is made from InDesign, the time to get these done is during PDF creation, not later.

[xpertpse]

Thanks for your prompt reply.

Please let me know which kind(s) of flattening you need - transparency and/or layers.

Also, do you need a particular version of PDF (e.g. PDF 1.4) or of PDF/X (e.g. PDF/X-3)? It would be helpful to know why you have this requirement."

The flattening of the layers is needed for sure. The print office has also mentioned that the PDF should not have any transparency layers. For version compliance, they are ok with >=1.4, no specific PDF/X or anything else required.

The PDF fils are not being created from InDesign. They are being generated from PDF library.

Please let me know that how to make the PDF flattened from Acrobat. Also, what are the settings that i should choose to make sure that the images and artworks remain in the required resolution. The things I dont wanna loose are the proper resolutions and the appearance of the layout after flattening.

[Dave Merchant]

Unless there's something strange happening with the inks, your best bet is to save as PDF/X-1a:2001 - that doesn't allow layers or transparency, ensures fonts are embedded, and will also push all your objects into CMYK.

You can either re-output from your original app, or use the PDF Optimizer tool in Acrobat Pro. If there's a reason you need other color spaces, you can configure the Optimizer to flatten transparency and remove layers (OCGs) without affecting the plates; though there will inevitably be some resampling of bitmaps when transparency within their bounding boxes is removed. The Optimizer is accessed via the Save As menu.

[Test Screen Name]

I had thought about using PDF/X. I certainly wouldn't use PDF/X-1a if there is any RGB; PDF/X-3 may be OK, but you do need to know what the output intent is, or you can end up with unexpected colour shifts, especially if (not recommended) the file used device colour spaces. If an ICC blend space was used for transparency, that can safely be used as the output intent.

Flattening layers is a trivial operation; the layers aren't separate as in Photoshop. Rather, each thing in a PDF might have a marker putting it in a layer. Stripping out all these markers leaves you without layers, and with everything visible.

Flattening transparency is more problematic. However, it has to happen every time you print, one way or another, so it is well tried.

Flattening will change some of the elements in the file, turning some things into curves, some things into rasters - anything that looks the same. Here is where the quality comes in. These NEW bitmaps have to have a suitable resolution. Images are not the main quality problem - text is far more of an issue. It is not a thing to do with a PDF for interactive use, as the text can degrade on screen and become unsearchable. But the transparency flattener has quality settings (you have the same settings in the print dialog).