resolution and file size

[suninthesky]

i have a problem: i want to reduce the size of a pdf, so i set a lower resolution and the app says the new file size is like 10MB, but then when i save on my computer (mac) the image it becomes like 22MB... anyone can help?

[c.pfaffenbichler]

Was that vector data originally and does it contain text? 

Because the damage might be relevant. 

What are the pdf settings? (A CMYK pdf might be noticeable larger thatn an RGB one and if you »Preserve Photoshop Editing Capabilities« then the issue would be your own choice…)

As for the size: When you save a jpg-copy via Save As and one via Save for Web (without metadata) is there a significant size-difference? If so the issue might be Ancestors Metadata. (Which could be removed with a Script.) 

[JJMack]

File Size is more related to the number of Pixels and the amount of details in an image. For Data compression is use to compress the image data int files. Resolution is just a printer setting to set the size the image pixels will be printed by the printer.  Changing the print resolution setting does not change file size.  You need to resample the image to reduce the number of pixels(change canvas size).  The are also different types of PDF files Photoshop PDF files are Photoshop images files layered like PSD files.  Acrobat PDF files are print document that have Pages that may include images. You import Pages and Image from Acrobat PDF Files. Photoshop also saves  PDF presentation files which contains a sequences if Photoshop Images, The may be flattened.  Layer will also increase the size of image files that support layers. So File format Plays a big part when it cone to File size. Photoshop edits Documents it not a file editor. It save files from Photoshop Document open in Photoshop and different file formats have different support.   Photoshop Native file format would be PSD or PSB. Other file types may not have support for all of Photoshop features. Common Web formats like Jpeg and Png file do not support Layers and all Image application  do not support layers. For example Lightroom does not support layers in Image file.