There is a maximum file size of 15 mb for uploading to Family Tree. Large PDF file sizes are easy to create, especially if you have scanned you documents at too high of a resolution, or if you have many "image" files inserted into the document.
Don't worry, there are some easy solutions for these large file sizes that will shrink the file size down easily. Some free online PDF sites that will compress your PDFs are noted below:
The links above constantly change as the programmers continue to improve the site, and then sell it off to another company. Some have begun charging a fee for their service after a few free trials. There is a great need to be able to combine separate images into a single PDF document. Search for "compress PDF" to see what other sites will let you do this for free.
If you are a Apple Mac user, you can simply select all the separate scanned images, right click and "Open in Preview". All of the images are now packaged into a single file. To save the file, click in the menu "File" --> "Print", and then save as a PDF file. (This option is near the middle and bottom of the Print Dialog box.) If your file is too large, there is a way to compress the file, but it is a bit quirky and often doesn't yield good quality pages, especially with images. I personally use an app called PDF Expert in the App Store. It can combine pages to a single file and also produce good quality pages after file size reduction.
If you are a Windows user, you will have to explore for PDF apps that are in their Windows Store. Adobe Acrobat from adobe.com is a well known PDF app that has many different functions and abilities to manipulate PDF files. It is however, an expensive app to purchase or license at $19.99/month. I would look at other options personally as you only need a few basic functions (combine and compress). A search for Windows PDF apps brought up quite a few that will create the PDFs, but do not have the COMPRESS feature.