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The file in PDF format created by Photoshop is mixed with various information and becomes very heavy.
I make a note how to reduce the size of photoshop PDF file.
Open a PDF file in Photoshop and "Save As" new file
Open the PDF file in Photoshop and "Save As" new PDFfile.
File > Save As
At this time, uncheck unnecessary options (such as save duplicates,alpha channel,layer information etc...) on the save screen.
Even with this, the PDF file size will be slightly reduced while maintaining the quality to some extent.
Uncheck unnecessary options in PDF Save Options
And if you want to lighten the PDF even more.
When you save as the PDF file, you will be see with a PDF save option like the image below.
If you want to save PDF file size lighter quickly, select "Minimum File Size" from the Adobe PDF presets section of the "General" tab.
Also, uncheck the options you do not need in the options column.
In my case, the 4.53M file that saved the entire Web page as a PDF was reduced to 258 KB.
If you don't care about the image quality, it will be much lighter.