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Preflight

Quicklinks: acrobat-settings, acrobat-preflight, acrobat-output-preview,
indesign-link-info, pitstop-preflight-profile

We only accept standard PDF/X-1a compilant files.

Review with Adobe Acrobat Professional your PDF files before sending.

Do not review with unconfigured Adobe Reader, because of the lacking of the possibility to see missing trim/bleed issues (no visible bleed) and faulty overprint problems (showing only yellow separation).
Do not create PDFs with Acrobat PDF preset "Standard". This creates a PDF files to be printed to desktop printers or digital copiers, published on a CD, or sent to a client as a publishing proof. This set of options uses compression (low quality image of JPG) and downsampling (low resolution image of 50 dpi) to keep down the size of the PDF file.

Correct your settings in Adobe Acrobat

Please see this guide how to configure "Show trim boxes" for seeing missing bleed issues.
Read more about PDF page boxes (prepressure.com).

Please see this guide how to configure "Overprint preview" for seeing faulty overprint problems.

Preflight your PDF in Adobe Acrobat

Acrobat: Preflight your PDF with tool Preflight (adobe.com).
The suitable preflight profiles are Prepress - Sheetfed (CMYK) and Web offset (heatset, CMYK).

More about preflighting: prepressure.com

10 reasons why should preflight: enfocus.com

Preview and check your PDF files before submitting

Acrobat: Overview of Print Productions Tools - Output Preview (adobe.com)
Acrobat: Preview color separations: disable preview of black ink (adobe.com) (screenshot)
Acrobat: View color warnings: enable view of rich-black (adobe.com) (screenshot)

Acrobat: Output Preview and Color Conversion (layersmagazine.com)
Acrobat: Prepress-Perfect PDFs with Acrobat 8 Professional (layersmagazine.com)

More guides

InDesign: Link Info - how to check effective resolution of images (adobephotoshopsecrets)
Photoshop: change the resolution - how to resample/upsample your low-resolution images (graphics.com)