The nature of the PDF can carry a lot of weight. There's no real solid guide to what exact repercussions may be without knowing the original application which generated the PDF and the setting (job options) used when creating the PDF itself.
Repercussions of opening a PDF which was not created by Photoshop in Photoshop:
- All art is rasterized. You lose sharp, crisp, vector art and gain pixel art with anti-aliasing.
- All Text is rasterized - no more text hinting, no vector text outlines, everything becomes a pixel. There will be no "live" text, so you can't simply alter text with the type tool. You have to treat text as art, because that's what it will be.
- No layers -- all art is one layer. This may or may not make editing difficult.
Essentially if you open a non-Photoshop PDF with Photoshop you convert the PDF to one big image. This may mean the overall file size will increase, sometimes dramatically. And it almost always means editing is much more difficult.
It's generally a bad idea to rasterize an entire PDF by using Photoshop. There are better applications out there to edit a PDF, the obvious being Adobe Acrobat. Photoshop is not designed to edit PDF files.
However, if you are seeking a JPG or PNG of the PDF, then using Photoshop is fine and shouldn't be an issue at all for that particular need.
If the PDF is a Photoshop generated PDF with "Maintain Editing Capabilities" set to on:
- Any loss would depend upon version and feature sets. If you save the PDF from Photoshop CC and open the PDF with an older version of Photoshop, some areas may be flattened or expanded to maintain appearance. This is similar to opening a new version .psd with an older version of Photoshop. It all depends upon what features were used in creating the file.
- Fonts. If the original file uses fonts not installed, you may run into some font replacement issues.
If the PDF is a Photoshop generated PDF without "Maintain Editing Capabilities" set to on:
Some applications save multiple file versions within the PDF wrapper, and that's all PDF is - a wrapper. For example if you save a PDF using Adobe Illustrator with "Maintain Editing Capabilities" set to on, you actually save both a .pdf and a .ai file all within one PDF file. If you open that PDF later with Illustrator, the .ai portion of the file is read, not the pdf portion. The same holds true for Photoshop -- if you save a PDF from Photoshop with ""Maintain Editing Capabilities" set to on you save both a .pdf version and a .psd version all within a single PDF file (wrapper). Opening that PDF later with Photoshop reads the .psd portion of the data.
Also realize the version of the PDF will play a large role in how inherently editable any PDF may be. Saving to PDF1.3 (Acrobat 4) will flatten all artwork and you'll lose all transparency and layers. Saving to PDF 1.4 (Acrobat 5) will maintain transparency but not layers. Saving to PDF 1.5 or better (Acrobat 6+) will maintain both transparency and layers.
In the end..... it all depends upon the PDF and what application generated the PDF.