Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
Your pdf file is a vector file and has no size. It is controlled by setting the density before reading the pdf, which defaults to density 72. So the vector file is converted to a raster at low density. Going from vector to raster will definitely increase the file size. To get better quality, you should increase the density before reading the pdf and then if that is too big an image (WxH), then resize down by the same ratio of 72/density.
Perhaps you should provide your input PDF file so we can test with it. You can upload to some place such as dropbox.com and put the URL here.
Please, always provide your IM version and platform when asking questions, since syntax may differ. Also provide your exact command line and if possible your images,
See the top-most post in this forum "IMPORTANT: Please Read This FIRST Before Posting" at viewtopic.php?f=1&t=9620
I had some attempts to work with PDF files, and I must say that it is laborious and long work.
Not long ago I started using the online PDF file editor https://edit-pdf.pdffiller.com/ and now it takes only a couple of minutes to edit the document, you should try this tool and evaluate the benefits of using it as I did.
It worked. Well, the same procedure applies if part of the corresponding editing should be included a data file.