- Do you need that size? What is the real physical size you need this document for? (And no, a billboard is not made at 300 PPI)
a lot of pictures
- So, you do not need PNG, you need JPG with good quality. Probably TIFF with good compression settings, or a flat PSD.
Too late did I realize that using PDF would be much more convenient (than PNG)
- The same. No. PDF is not just one format, there are several configurations that will make a project suitable or not, including resizing the image to the physical size (Point 1) and making internal compression of images as JPG (Point 2)
In the best case scenario not losing quality.
- The best scenario is making things the right way from start... Back to point 1. What is the real size of the project?
I. Define the real size you need for the project. (A quick tip. You almost never, ever, need a file that is larger than 12,000 px in the longest side. Normally with 6,000 px on this side is enough)
II. Flaten your PSD... I have not worked with that size, if flattening all the layers at once cannot be done in your computer, try merging some layers step by step.
III. Resample this flat image to match point I. Use Bicubic Sharper.
IV. Save as PSD. Most people on the planet can use this image for print. You can also try a good quality JPG.
V. Avoid using PDF for this project. As you do not know the additional configurations needed.
A PDF file is when you prepared a vector file and you have vectors, and pictures (raster) images. But if you only have one humongous raster image, a PDF file is just one PDF with an image inside... so send the image alone (JPG, PSD, TIF)