Thin white lines in a PDF may be transparency-flattening or display stitching rather than actual gaps in the artwork. Test at several zoom levels, in a suitable second viewer and through the printer's output workflow before repainting the image. Some lines are real defects, so a screen explanation alone is not sufficient approval.
Locate the lines in the actual PDF
Open the exported file, not only the layout preview. Note whether lines follow boundaries where transparent objects, images or vector regions meet. Compare several zoom levels. Lines that change prominence or disappear with display settings suggest a rendering issue, but that pattern should be checked against output.
If the same gap is plainly present in the source image, investigate the artwork instead. If only one export has the lines, compare its compatibility and transparency settings with the intended delivery requirement. Avoid changing unrelated picture resolution before identifying the stage.
Respect the printer's accepted workflow
Adobe explains that flattening can divide artwork into raster and vector regions, and documents thin-line stitching in certain exports or viewers. Later PDF versions can retain transparency, but the appropriate delivery depends on the printer's requirements. Ask whether it accepts a transparency-preserving file rather than selecting a new version blindly.
Keep the editable layout, make a new export copy with agreed settings, and compare it with the current delivery. Do not overwrite the only working file or assume a flattened PDF can later recover its live transparency.
A hypothetical photo-poster proof
Imagine a poster with a photograph, translucent title panel and shadow. Fine white boundaries appear around portions of the image at one screen zoom. A different viewer renders them differently, while the printer's proof does not show them. The designer records that result and approves the verified output file.
This is a hypothetical procedure. If the printer's proof also contains the boundaries, the designer must investigate export or output settings instead of calling the lines harmless. A screenshot of a clean preview cannot replace that check.
Avoid fixes that damage the design
Painting over every screen line can create visible stripes in an otherwise intact image. Rasterizing the entire layout can reduce text quality or eliminate useful print structure. Use such changes only when they solve a confirmed output problem and match the delivery agreement.
Imagild may supply the artwork, while PDF assembly and prepress inspection happen in an external layout tool. A high-resolution generated image does not certify the exported PDF's transparency handling.
Acceptance record
Record the exact file and export settings checked, the viewer or proof used, and whether the lines were observed in output. Require intact lettering, image detail and intended transparent effects. When only the display shows a seam, document that limitation; when output shows it, repair and recheck. The goal is correct delivery, not making every arbitrary screen zoom look identical.