Issue with PDF files in iBooks

I like to keep various PDFs in my iBooks for a quick reference and general information. I have no problem viewing them on my MAC mini desktop in my 'all books', and then I can move and view them in one of the categories I have set up on my MAC. However, these PDF files are nowhere to be seen on either of our iPads!!!

I did at one point have the iBooks option ticked on iCloud, but found both my husband and I were finding it confusing to see where all the books were as some were in the cloud and some in iBooks so I brought everything back to the main iBooks program on the Mac mini, and now everything seems to be visible with the exception of the PDF files which are quite visible on the MAC mini but nowhere to be found on the iPads!

Can someone suggest what it is that I am not doing correctly to make this all work? I particularly like having these PDF files available to me while travelling etc. as it seems to work better for us. Have tried the iCloud to store them but if we have no internet, this doesn't work. Also have used Dropbox and by ticking the document it is visible without WiFi, but the all around best solution for us would be to keep them all together in our iPad iBooks - we can then just pick out the PDF's that we are interested in to store in our individual 'category', i.e. DJ books.

Would appreciate any suggestions!

Thank you,

Gayleen77